Note: I have a separate page dedicated to retailers of hoof boots so you don't have to wade through the general equipment list below.
I have also listed general BDSM equipment retailers, and eBay listings for ponyplay related gear on their own pages so as not to confuse things.
Fury Fantasy - Gorgeous and realistic looking pony heads. They sell most everything pony play related: bridles, hooves, harnesses, and masks. If you have not already heard of them, it's worth taking at look at their gear: they have some phenomenal stuff.
Sinvention - Canadian leatherwork company with beautiful, high quality BDSM gear. They have a fairly wide selection of pony play gear under "The Stables at Sinvention" section. There is just something about their gear that I love...can't put my finger on it though. Note: They have a couple items of ponyplay related gear which are not listed in the "Stables" section. Most notably their hobbles and pony arm splints.
Fleet Ilya - Beautiful, high quality harnesses, saddles, and masks (they have dog, bunny, and fox masks). Their ponyplay saddle (available in black, chestnut brown, and "nude") is actually one of the nicest designs I've seen. However, at £3000 it costs nearly as much as a nice bio-horse show saddle from Hermès. They also have a few more "moderately" priced items. It's worth checking out their harnesses and masks even if you don't want to buy.
Leder Gummi Studio - They have some pretty cool pony play and puppyplay gear including hoof boots, bridles, and masks. You may want to go directly to the page on ponyplay).
Punitive Shoes - They have some nice looking pony boots with different color and length options. I have heard that they can be somewhat slow in shipping, however, I purchased a pair of their "Derby" boots a couple years back, and they did ship within the 45 business days quoted. Also, they have switched to a handmade horseshoe in place of an actual horse iron due to shipping costs. This change is not reflected in their product photos, so the horse shoe you will receive may be different than what is pictured.
Marquis - Fetish wear and some BDSM stuff. They have a few pony items: bridles, pony gloves (hoof mitts), and foot trainers. It looks like they discontinued their pony boots, which is a shame since they were actually pretty hot looking (as of 2012-12-10)
S-Art - A lot of pony items. They have hoof boots, saddles, sulkies, tails, and more.
Mise en Cage - They have some really cool looking pony gear especially the metal plug tails that come in black, blonde, or grey. They do also have some head gear complete with pony ears. The price is actually not that bad if the quality is on par with what is pictured.
Locked for Infinity - They have a beautiful, elaborate pony play harness that is based on JG Leathers' "The Creature" (seeing a woman wearing the JGL pony harness in a photo was my first exposure to pony tack desgined for humans. When I first saw the tack, I searched web pages, message boards, and magazines for a long time looking for his contact info (this was, of course, well before Fetlife and such). I was exhilarated when I finally got in touch with him, I ordered my very first bridle from him (which I still treasure and use today)).
Fetish Universe - The site is mostly in German, but definitely worth taking a look at. You may wish to go directly to their page on pet play. They also sell tails (they even have a glass butt plug tail), nice hooves for hands, bridles, and really hot looking pony play saddle. Cool stuff.
Fantasy Leather - Superb ponyplay harness at a (relatively) reasonable price. They will also work with you for custom work in leather or rubber.
Fetish Zone - They sell hand hooves, horse head masks, bits, and dildoes (including the wearable Pegasus). Fetishzone also carries a great selection of puppy play gear.
Wolf Princess Designs - A zebra head harness, tail, and zebra strap-on as well as pupppyplay and kitty play stuff. They even sell a bunny strap-on! Pretty cool stuff if you like exotic animal play (e.g. zebra, fox, wolf, tiger, etc.). They have some really cool bridles and harnesses (their front page images rotate) which I believe are custom work they have done previously.
Wild West Leather - Based in Seattle, WA, USA. They sell a beautiful ponyplay harness (and includes a bridle), which is a little on the expensive side ($1000 USD as of 2012-11-06), but looks amazing.
Lederstern - They sell hoof boots, a ponyplay saddle, and some paw mitts. The black leather hoof boots are pretty cool looking.
Zeta Creations - Dildos...in some unique shapes. Although they have discontinued the Thor dildo (at over 2 feet long - yeah, that's over 24 inches - that one was a monster), they do have some pretty interesting toys.
Bad Dragon - Although they specialize in dragon toys, they have anatomically correct dildos for several species including horses: they have both fully flared post-orgasm, and unflared dildo molds from a Thoroughbred stallion, "Chance the Stallion."
Native American Exotics - Some really nice bridles and other ponyplay tack. Their tack has a Western look to it, which is not really seen with most pony play equipment. They also sell hoof gloves and pony carts, and their prices are quite reasonable.
Pony Play Ranch - Good selection of pony hooves!
Terginum - They have some pony gear including a bridle, which looks pretty decent and seems to accommodate standard bio-horse bits (including bits with curb action), which is always a plus in my opinion.
BDSM-Laden - They have a nice ponyplay head harness and glass tail butt plug. The site is in German so you may want to go directly to the ponyplay and petplay section.
Abigails Stable - They sell a nice bridle for pony play. It comes with mane, plume, blinders, and ears, and your choice of bit. They have several color options as well. In addition to the bridle, they also sell a complete harness for pony play.
Truly Heavy Leather - Pony play harnesses custom fit to your pony. The price is more than reasonable (especially given that the price for the pony harness also includes a bridle), and the harness will last - these harnesses are real working pony tack.
Fetish-Pets-Store - They have gear for ponies, dogs, cats, and pigs. They also sell hoof boots.
Restrictionwear - They have a nice selection of good looking and functional harnesses and bridles, but they do not list prices for most pony play products on the site :( ...you will have to inquire for most pony play equipment because they make it to your mesurements. They also do completely custom work and will work with you to design your own look. Look under their ponyplay section or search "pony"
9 mm SFX - They have a wide selection of theatrical contact lenses. While they do not (at this time 2011-09-09) have contacts emulating equine eyes, their goat eye is pretty cool.
FX Eyes - They offer a good selection of theatrical and special purpose contact lenses. As of the time of this writing (2011-09-09) they do not offer contacts emulating equine eyes, but they do offer custom theatrical designs to your specifications.
Spandexwear - Spandex suits for various animals including ponies. They also have other equine suits such as zebras, and even unicorns. The suits come with options such as an added mane.
ProDeviant - Fetish and erotic artwork, leathesworking and design. They make items for both ponyplay and petplay. I have not ordered from them so I can't comment directly, but I do know of a couple ponies who absolutely love their work. Edit: They are currently (2012-11-06) taking a break from production, but they do have a waiting list available.
Spanking Good - Has a new ponyplay section with great handmade gear with many options for customization.
Water Hole Custom Leather - Buck has some high quality pony gear, and if you want custom leather work that will last you a long time, this is the place to go. Buck's quality is extremely high, and you can expect his gear to last (I still have some stuff I bought from him over a decade ago that has seen a lot of use and is still great condition).
The Stockroom - Sells a wide variety of BDSM gear including some pony play tack. They also sell some cool puppy fist mitts and other pet play equipment. I've purchased a few items from them over the years, and they are a great place to find a wide variety of BDSM gear in one place for a decent price.
Dover Saddlery - A large non fetish english tack store with lots of horse related products. They are especially good for saddles, girths and bits. They also sell tongue curbs, bit burrs, tongue straps and other items that can be used (or easily modified to use) on a ponygirl or ponyboy.
State Line Tack - A large non fetish tack retailer. They sell english and western tack, and other horse related items.
Dressage Extensions - A wide variety of tack. They have a nice selection of spurs, whips, and saddle pads.
Bit of Britain - An extensive selection of tack. Not just bits, but they have those too :)
Human Ponyplay - A frequently update photo blog on ponyplay with a selection of cool photos and narrative. This is one of my favorite ponyplay sites. Check it out!
Fetish Pony - Suede's website on ponyplay. This is a wonderful site on ponyplay with several videos demonstrating the gaits of a ponygirl.
Sir Jeff's Ponygirls - A free ponygirl website with a huge (and I really mean huge) amount of pictures, stories, and other very fun and useful resources. There's really not much else to say except this is arguably the ultimate ponygirl website.
The Human Pony Registry - A little like a bio-equine breed registry (e.g. The Jockey Club for Thoroughbreds) for human ponies. The Human Pony Registry is not limited to ponies (i.e. trainers may register as well) and describes itself as: "...a group of dedicated people interested in the Pony play lifestyle. A place where Trainers, Owners, and Human Ponies can come together to meet and find others." They also have a nice article on ponyplay tack with advice on creating tack items from inexpensive hardware items - great advice for potential ponies and their trainers!
My Ponygirl by Sir Anthony - This is a really cool blog. When I first put the link here, I'll confess to only scanning through and checking out the photos, but after spending a few minutes and reading his posts, I'm really impressed. It's definitely worth reading.
Mystic Storm and Luna - The pony and trainer pair were awarded the title of International Pony and Trainer at the 2007 Florida Fetish Weekend. Mystic Storm is also the creator of the pony pride flag, which serves as a symbol of pride and involvement in the ponyplay community. The site explains the meaning of the symbols and coloring on the flag as well the inspiration for its design.
Ponyboy Rusty - A blog detailing the thoughts of Ponyboy Rusty. Interesting reading coupled with nice photos makes this is a nice blog on ponyplay from the perspective of a ponyboy.
Ponygirl Drafter - Pony Girl Drafter is a "nomadic human Clydesdale" who I know is quite active in the Fetlife ponyplay forums. There are some photos, links, and a nice blog. I think she also runs the informational Alternative Pony website, which, while still under construction, is unique and worth a look becuase she discusses western ponyplay (most ponyplay sites discuss english style ponyplay).
Danny the wonder pony - His official website has several photo galleries documenting his long history of being ridden in various clubs. His story makes for quite an interesting read: apparently he has made a few television appearences and even been in a Quentin Tarantino movie!
Chargerpony - Charger's personal site includes several photos of him tacked up (he has some pretty cool looking gear) and being ridden. He also talks about growing up around horses and how he became interested in becoming a ponyboy.
Silver Sparks - Silver Sparks has a great informational site on ponyplay with some cool pictures.
Pony Unique - This is pony Unique's site (based in central Texas). She has some pony play and BDSM links and a couple pages of photos of her as a ponygirl (she makes a beautiful ponygirl). Although the site is listed as under construction, I don't think it's been modified for quite some time.
Ponygirl Lyndsey's Corral - Ponygirl Lyndsey's personal site. Quite a few photos along with some useful information for those interested in pony play.
Rubber Ponygirl - Ponygirl Shyanne's personal site based in Australia. She is active in the pony scene in Australia (and runs a couple groups on Fetlife), so if you're local to her, it might be worth checking out her site and/or getting in contact with her.
The Stampede - The San Francisco Bay Area Animal Roleplay Society. It's not just for ponies: anyone interested in any type of animal roleplay is welcome. If you live in the bay area, drop by a munch (munches are on the second Tuesday of every month from 8PM - 10PM) at Wicked Grounds Kink Café.
Bridgewood - is an open-air BDSM and ponyplay friendly venue. It is a 20 acre woodland in North Shropshire between Manchester and Birmingham and is privately owned by Tanos. The site is nicely done with an extensive, well-researched account of the history of ponyplay.
L.A. Pony and Critter Club - The Los Angeles pony and critter club ("Exploring human pony and critter role play in BDSM") is run by subMissAnn (aka Beauty) and welcomes anyone interested in animal roleplay - just like The Stampede, it is not just for ponies. If you live in southern California, drop by one of their munches or many play events.
Wingnut Stables - They are a BDSM and pony play friendly 17 acre private facility in central North Carolina. They require membership before you are allowed to attend any of their events, and membership is by invitation only. They suggest that you attend public events in the Raleigh area if you interested in membership.
The Other Pony Club - "The largest club in the world for those who want to take a "bestial" role and their fans. Pony-girls, pony-boys, puppies, piggies, cats, zebras, anything you may want to be." They are based in the UK and hold various events.
Renegades - This club for ponyboys is based in New York. It is primarily focused on gay male ponies, which I think is cool because I seldom hear much about male pony play (more often it is puppy play).
TEC @ TES- The Equestrian Club at The Eulenspiegel Society™ is based in New York. It is mostly for pony play, but they are also open to those interested in other animal play.
West Coast Pony Players - "The official newsletter and discussion group for myponygirl.com. A friendly place for fans of the human equestrian fetish."
RMequine PonyPlay - Ponyplay group for those in the Rockies.
Stampede Talk - "Open discussions on topics of interest to The Stampede - The San Francisco Bay Area Human Animal Roleplay Society"
PonyPlace - A group for discussion of ponyplay open to anyone interested in ponyplay, but they are based in southern California.
PonyGirls1 - A group open to anyone interested in pony play, puppy play or kitty play.
Sir Jeff's Ponygirl Group - "For the lovers of good adult ponygirl pictures, drawings, art, stories."
So Cal Stables - A southern California ponyplay group.
Midwest Area Ponyplay - Ponyplay network for the Midwest.
Equus Eroticus - A group that keeps members informed about Equus Eroticus magazine and to take suggestions for stories, features, etc. as well as general ponyplay discussion.
Pony Play - Discussion and exchange of information on ponyplay.
Pony-Paradise - Discussion of ponyboys and ponygirls.
Pet and Ponygirls - As the name implies there are puppies, bunnies, cats, the occasional pig or zebra, and of course, lots of ponies. Frequently updated (a few times a week) with lots of pictures and videos.
Kinky Ponygirl - Lots of pictures updated 3 times a month. High quality, original content. Quite a wide variety of ponyplay scenes with some really elaborate bondage. The site is definitely BDSM oriented with some strict ponygirl training scenes, and they even incorporate some eletrical play on occasion.
Pupett - Pupett makes a cool ponygirl. There is some free stuff, but some content requires payment. If you have read "The Human Pony", you will know she features prominently in many of the book's photos. While she is a professional fetish model, she really does get into the ponygirl role.
Ponygirl Master - Videos and images of beautiful ponygirls being trained and ridden by Major Syn. There are some really stunning photos on this site. It's definitely worth taking the free tour.
House of Gord - They have some ingenious and strict training devices and regimens. I don't subscribe (not because I don't like it - I simply can't rationalize paying for subscriptions to a bunch of sites), so I'm not sure how much you really get with a subscription, but the sample photos and videos are pretty cool.
Shadow Players - Photos and videos with a fair amount of ponyplay content. While not exclusively pony play, one of their models was a cart ponygirl at the Folsom Street Fair.
The Human Equine - Trigger's website where he describes his interest in pony play. He has several videos available for purchase in which he is carrying female riders.
FetLife - A large BDSM online community with many ponyplay and human pet discussion groups. It's a little bit like facebook for kinky people. If you decide to join, I recommend the Pony Play discussion group, which has over 4000 members. You can also find me on Fetlife as cpony - stop by and say hello.
The Veterinarian - He makes wonderful sketches of animal role play: ponygirls, puppy girls, and more. See box 11 of this list for much of his great artwork.
Wicked Grounds - If you live in the San Francisco bay area, or are just visiting, stop by this kink café and boutique - it's the only place in SF to get coffee served in a dog bowl.
Bondage Awards - They give out annual awards for bondage sites in over 50 different categories. Aside from being a fun competition, I found some cool sites I probably would not have otherwise discovered.
Pony Play Links - A great compilation of links specifically for pony play.
iTaboo - A BDSM social network and dating site for alternative singles: "A FREE BDSM Social Network for Adults with Deep Taboos".
Pony Play Latin America - Informational articles and discussion forum on pony play specifically for those in Latin America (site is in Spanish).
Equus Eroticus - Originally a print ponyplay magazine (founded in 1997 by Paul Reed), it transitioned over to an online publication run by Rebecca Wilcox in 2009. The site has some nice articles and interviews in addition to a discussion forum. The original 22 print issues of Equus Eroticus are not available in electronic form, but they can be purchased from the website.
Hooves-R-Us - An illustrated step by step guide to making your own pony hooves (for hands).
Make Your Own Pony Hooves - A guide to making your own pony hoof boots written by "Boots" the ponyboy.
caryl's Page - Pony and pet related links
Rubber Dawg - Dog and puppy play masks for various breeds of dogs.
Dutch Pony - Dutch ponyplay site.
"Pervy Ponygirl" - A nice article on pony play. I love that the author's name is Tristan!
Kitten Cream - Extensive selection of kitten play gear including kitty play tails, ears, and collars. Only kitty play (not ponyplay), but I stumbled across the site and their gear looks really nice. Coupled with the fact that kitty play gear can be somewhat scarce, I thought it was worth including them here.
Harriet Holt Pony Boots - Pony boot designs by Harriet Holt inspired by "fetish" animal roleplay: "I designed a range of footwear for 'human ponies' which embodies the elegance and majesty of the equine form and the complex power dynamic of the human-equine relationship; the wearer feels strong and beautiful, yet is physically fettered and utterly at the mercy of another." Some pretty cool designs, but I don't think they are available for purchase since these were prototypes exhibited by design students. More photos of her therianthropy boot designs.