There are several manufacturers of boots, here is an ad for one: Easyboot Epic EasyCare, Inc (Equine - Supplies Tack - Sports Medicine - Barefoot Boots)
Price, quality, ease of use, etc vary from brand to brand. The good thing about them is you put them on and off yourself as needed. Most horses can do with just the front feet booted up but some need them on all four corners. In the long run they are cheaper then shoes.
You do want one with some type of gator like the one in this ad to keep from losing them. They can step them off or get them sucked off in the mud. The gator keeps them attached to the leg after they fall off. Bright colored ones are also easier to find then black.