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 2020-01-12 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 - €2789 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).
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 - €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)
Turnips And Pixies - $395 CAD: A cool pair of hoof boots with a realistic looking hoof portion for a decent price.
Horse King - $750 - $1500 USD: 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 transitioning from Etsy to their own website, so you should contact them directly (e.g. on twitter) if interested in placing an order in the interim.
Hot to Trot Costumes - $399 USD: Realistic heelless hoof boots with easy zipper entry (the zipper is hidden to maintain the aesthetic appeal). The fur colors are fully customizable and the boots are made to order. The boots are shod with a genuine horseshoe. Orders appear to custom, so contact them for details.
Birch Place Shop - $239.20 USD 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 £161.83, 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).
Magpie Bones - Amazing work and the hoof boots start around $200. They state they will have openings for custom work this month (January 2020), so 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.