Hoof boots are one of the most popular pieces of equipment in ponyplay. Here are all the retailers of hoof boots for ponyplay I know of. Price data is accurate as of 2015-11-16 unless otherwise noted.
Clicking these links will take you directly to the retailer's hoof boot page or section of the page. I've tried to put a price when possible.
Fury Fantasy - €1449.00 EUR: Very high quality boots with lockable zippers as an option. They also have cool hoof gloves as well (€989.00 EUR for the hoof gloves). Note: Temporarily unavailable while they are redesigning the boot. Update: They have announced a scheduled release date of October 2015 for the new hoof boots (as of 2015-11-16 they have not as yet released the updated boot, but it should be available very soon. If interested in the new boot, you can contact Fury Fantasy to place an order).
Lederstern - €720 EUR: His pony boots look pretty cool, and you can get them in black leather, brown leather or detachable faux fur. They also come with a horseshoe. The look of his boots is closer to the Reactor/Kayser look, where the foot actually goes down into the hoof instead of resting on top of it like a platform (though this may make it slighter more difficult to walk in).
S-Art - €799.00 EUR: Made from leatherette, these boots area reasonably priced and look pretty comfortable (though I do not own a pair and thus cannot say for sure). They have a definite hoof shaped bottom, but it is unclear if they are shod or not (one product photo shows the outline of a horse shoe whereas the second product photo does not).
Leder Gummi Studio - €799.95 EUR: Their boots resemble the old pony boots of Marquis with the shiny metal accents on the hoof portion. These are a little more on the expensive side, but paired with one of their metallic bridles, your pony will have a unqiue look that's sure to turn heads.
Fabulously Fetish - £630 (£525 EX Tax): The Epona hoof boot by Fabulously Fetish is a realistic looking hoof: from the light black hair to the hoof itself, the boot is well made and would make a lovely addition to pretty much any pony outfit. It is a heelless boot made from a solid wood platform that is completed with a steel horseshoe. I don't own a pair personally, but I like the look of them, and, coming from the maker of these breathtaking reindeer costumes, I have no doubt about the quality and aesthetics.
Punitive Shoes - €269.00 EUR: Their hoof boots are nearly ballet boots (which I personally like) and the quality is high. Price is for the knee high version of the hoof boot though the prices are similar for all their styles (from €239 for the ankle boot up to €319 for the thigh version). Note: 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. If you want a genuine horseshoe, you can replace the new machined aluminum shoe with a genuine horsehose (they are attached with screws). I believe the horseshoe size is 6X0 (i.e. 000000 size)
Water Hole Leather - Buck has retired and has closed down his shop. However, he can still be contacted via his website.
Turnips And Pixies - $350 CAD: A cool pair of hoof boots with a realistic looking hoof portion for an awesome price! At well under $300 USD ($262.34 USD at current exchange rates), these are an incredibly good deal and look great. Note: The boot linked to here says it is not available, but I believe this just means that it is not in stock currently. I think the boots are still available. Just contact the seller and mention you would like a pair. However, I will update this note when I confirm this one way or another.
Birch Place Shop - £198.24 Available in black, red, white, or pink leather, these thigh high pony boots also come complete with four lockable straps to keep them in place. The heel-less platform design is constructed of hard foam and capped off with a steel horseshoe. Frankly, these are some hot looking boots at a very competitive price point! They also sell a PVC thigh high hoof boot for £151.83, which is likewise equipped with a steel horseshoe and is available in an even wider selection fo colors (though the PVC version only features one lockable strap at the top of the boot).
Ponyplay Ranch - $785 - $1210 USD They have a lovely hoofboot with lots of available options. The exact price will depend on the style of the boot and the chosen option, but expect to spend around $1000 USD.
Horse King - €535 - €700 EUR: A very nice looking heel-less hoof boot. The hooves are made from hard resin and the upppers are leather from Punitive Shoes. The uppers are available in black and red and the hoof portion is black. They also sell a cloven hoof version, which goes for around €500.00 EUR.
Magpie Bones - Amazing work and the hoof boots start around $200. They take commisions, but due to high order volumes, the earliest a new commision will be started is January 2016. You can check Magpie Bones' Fur Affinity profile periodically for updates on wait times.
Oonacat - $350 - $900 USD: Heelless hooves, horns, and paws. Currently no hoof boots are in stock in the etsy store, but for reference you can see various styles by this artist on deviant art. The price depends on the style, but ankle high hoof boots seem to have gone for around $450, which is fairly reasonable.
Wyvern Creations - Ponysweetcheeks makes realistic looking hoof boots to order. While he is setting up his web store, you can visit his Fetlife profile (you will need to login) where you can see photos of the boots he makes. If you like what you see, he is accepting orders - just contact him on Fetlife.
Do you know of a hoof boot maker/retailer not listed here? Let me know, and I'll be happy to add them.