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 2016-11-18 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 - €2498 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).
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). Edit: Their homepage loads fine but other pages are inaccessible. I think this is just a temporary technical issue.
Leder Gummi Studio - €672.23 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.
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. Edit: It looks like his site is now offline. However, you can check out his cool hoof boots thanks to the Internet Archive.
Turnips And Pixies - $370 CAD: A cool pair of hoof boots with a realistic looking hoof portion for an awesome price! At under $300 USD ($273 USD at current exchange rates), these are a good deal and look great.
Birch Place Shop - £196.05 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 £150.16, 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. The link goes to the Mustang hoof boot, but they also sell a Colt model and a Palamino model
Horse King - €860 - €980 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. Note: They are taking a short break but will resume accepting orders in mid-September 2016.
Magpie Bones - Amazing work and the hoof boots start around $200. They will resume taking new commissions on August 27th 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. Edit: I believe they mostly do custom work, so while boots may not be in stock, I recommend you contact them if interested and they can make a pair for you.
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.
Fabulously Fetish - I was troubled to find out about a fellow pony's bad experience ordering hoof boots from Fabulously Fetish, if you can even call it "ordering": he paid them and nearly 6 months later he said he received nothing. They would not even respond to his emails. Thankfully, he was able to to get his money back thanks to Paypal. However, I was horrified to find out that he was not the only one this happened to. After hearing from this pony and reading all the complaints, I have removed the link to Fabulously Fetish and am recommending that ponies steer clear of this shop. Edit: It appears they removed their Facebook page, which documented the negative experience of customers but are still selling boots on their website. I'm not sure if anything has changed, but I would advise caution if placing an order.
Do you know of a hoof boot maker/retailer not listed here? Let me know, and I'll be happy to add them.