To my knowledge, Horween Leather in Chicago (www.horween.com) is the only U.S tannery producing horsehide and shell cordovan. All of the horse-butts I have seen from other distributors (Siegel's, www.siegelofca.com) come from Horween.
As stated, horsehide is sold by the pound, not by the square foot like cowhide. It comes it strips that are generally 35-40 inches long, 6 inches wide at the ends and 12-18 inches wide at the widest point in the center. And it is not consistent thickness it's entire length; the ends are thin are more maleable which makes it tough to get in lengths long enough to kae good belts out of without requesting them. I also believe horween has a minimum quantity order so if you just want one or two butts to try out you may want to go with Siegel.
Also there are two varieties of horsehide offered by Horween (and Siegel) - hard-rolled and soft-rolled.
If you want it stiff you want the hard rolled type. Soft rolled is rather supple, more-so than cowhide. (Bruce, you may have had a piece of soft-rolled because the hard-rolled stuff is denser and firmer than even cowhide. I've had hard-rolled in 7-8 oz. that had less bend and give to it than 12-13 oz sole leather).
There is a general misconception, particularly amongst gun folks, that horsehide is thinner than cowhide. That is simply not the case. Horsehide can be gotten in just about any thickness that you can get cow in. It is just that hard-rolled horsehide is denser and firmer than cowhide, so a thinner piece can be used to accomplish the same project that may require a thicker piece of cow.
Do I think it is the be-all, end-all holster material. No, not necessarily. I don't see much advanatge to it for the main body of the holster. I do use it for IWB straps because it wears better than cow and doesn't soften as quickly. I also use it for reinforced mouthbands and thumb-breaks, to keep them stiff but thin in those areas. And yes it does make an excellent belt if you can get it in the appropraiate length. Another alternative is to use the horsehide as the belt liner with cowhide as the top slightly longer layer.