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 2015-03-31 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 (site says they are redesigning the boot) - accurate as of 2015-03-31. However, it is my impression that if you would really like a pair, they might be able to accomodate you.
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 - €259.00 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. 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.
Water Hole Leather - $295 USD: Buck has restarted production of both his hoof boots and fore hooves. His products are well made and reasonably priced.
Turnips And Pixies - $340 CAD: A cool pair of hoof boots with a realistic looking hoof portion for an awesome price! At less than $300 USD, these are an incredibly good deal and look great.
Birch Place Shop - £205.86 Available in black, red, or white 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. They also sell a PVC thigh high hoof boot for £143.90, 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 - $1200 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 - €400.00 - €615.00 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. However, it looks like all work is custom and due to the volume of requests, not all commisions are accepted (you can check Magpie Bones' Fur Affinity profile periodically to see if comissions are currently being accepted).
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.