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 2014-02-20 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 (€949.00 EUR for the hoof gloves). Note: Temporarily unavailable (site says they are redesigning the boot) - accurate as of 2014-02-20.
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 - €399.00 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.
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.
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.
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).
Ponygrl Wrangler - Not currently making boots, but may start making them again in the near future. He is the maker of the Wildfire hooves, which used to be sold on ponyplayranch.com. You can contact him on Fetlife under that screen name. His boots typically run around $1000 USD.
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.
Do you know of a hoof boot maker/retailer not listed here? Let me know, and I'll be happy to add them.