Hooves are one of my favorite items of tack. There are two basic kinds of hooves for ponyplay: hoof boots, and hoof mitts (gloves). Below I will briefly discuss both types of hooves and their use in ponyplay.
Hoof boots, as the name implies, are worn on the pony's feet. Hoof boots are designed to make the wearer's feet resemble horse hooves. This means that hoof boots do not have heels (see image below comparing a pony wearing a hoof boot to a horse's hoof).
Hoof boots are surprisingly common (given they are quite specialized) with several retailers supplying various styles (I have compiled a list of links to hoof boot retailers - the list has current prices and is comprehensive (as of 2015-05-16) to my knowledge).
There are quite a few variants around, but my personal favorite is the pony boot from Reactor Rubberwear of Australia (unfortunately they are no longer in business). This boot is remarkably comfortable to walk in (compared to other similar boots at least) and gives a very realistic impression of a horse's hoof. I also own the "Derby" from Punitive Shoes. The boots from Punitive Shoes are much less comfortable (although that might be the point), but they do force one to walk like a horse. I have adopted the system of using the Punitive Shoes hoof boots for schooling and the Reactor boots for shows. The image at the right compares the Punitive hoof boot to the Reactor hoof boot.
Hoof boots can be very dangerous so please use them with care. This is especially true for ponies with less training, ponies with foot or leg problems, ponies who are restrained, etc. It has taken me a long time to get used to these boots and learn to balance in them; even so, I still walk and trot with much more care while wearing them. I have found that they can be especially dangerous on inclines when on asphalt (found that out at Folsom).
Hoof boots are best suited to show ponies, but they can also be used with caution on cart ponies depending on the terrain. I would not recommend their use on riding ponies since the added weight of a rider can cause damage to the feet and ankles if wearing hoof boots.
Hoof Mitts (Hoof Gloves)
A hoof glove or hoof mitt is a bondage glove/bondage fist mitt that is shaped like a horse's hoof (see image below). Just as with hoof boots, they are designed to give the wearer's hands the appearance of horse hooves. They also serve the purpose of restricting the use of the fingers adding both an element of bondage and an element of realism.
Fore hooves, or hoof gloves, used to be somewhat less common, but in the last few years I have noticed that more specialty retailers now sell rubber hoof mitts. Leather hoof mitts are somewhat more difficult to find: the only retailers I am aware of that sell leather hoof gloves are Fury Fantasy and The Waterhole. Another option would be to use leather puppy fist mitts, which are more widely available (you can even add a horseshoe).
If you are not terribly worried about having realistic hoof shaped mitts, bondage fist mitts are widely available from most BDSM retailers. Even without the use of other restraints, high quality, properly fitted fist mitts can drastically reduce a pony's ability to perform many human actions, and they add a good degree of helplessness. Thus, I would recommend fist mitts of some kind even if they are not hoof shaped.
The widely available molded rubber hoof gloves (see image at right) are actually cool looking and quite durable. These hoof mitts are slip on. Once on, the wearer's hand is held in a fist at the bottom of the glove. The rubber will usually have some give, so it is usually possible for the pony to open its hand within the hoof glove (however it is hard to keep it that way due to the elastic nature of the rubber). Moreover, the rubber hoof gloves, when properly sized, fit snugly enough such that it is quite difficult for most ponies to remove them (provided the pony is wearing a hoof glove on each hand) without assistance. The molded rubber hoof gloves do not usually have any straps for tightening or attachment, instead relying on the elasticity of the rubber to prevent removal. However, wrist cuffs (locking or otherwise) can be placed over top of the gloves making it nearly impossible for the pony to remove them.
If you prefer to have leather hoof mitts, the only places that sell them (to my knowledge) are Fury Fantasy and The Waterhole. Alternatively, leather pony fist mitts can be made from puppy fist mitts via removing the "paw print" and addition of a metal horseshoe. This is exactly how I made my first pair, and I am reasonably satisfied with the result. Presumably, you could make further design adjustments to make the mitts more hoof-like (e.g. add in the details seen on the bottom of the molded rubber hoof mitts).