It's not common but it's hardly taboo in most places. The results of searching google for münchen pferdemetzger feature the Pferdemetzgerei in Munich's open air Viktualienmarkt.
Horse meat fell out of favour because we stopped needing to dispose of a lot of horses after the rise of the infernal combustion engine. Horses don't put on the kind of weight that cows and pigs do, nor at anywhere near the speed. Horse meat is also very lean making it less flavourful and somewhat dry. The muscle fibres are shorter so can have an almost powdery mouthfeel. Poorly raised/fed horses may be very fatty, resulting in well-marbled meat which is very nice. Overall, horse is specialty product and it's more expensive than the usual meats in stores, on par with top quality beef.
I don't know if you can get it inspected in the US. If you can't, then it can't be commercially produced. This severely limits your options. I have no idea where to turn in that case. I get it once in a while just because I'm going nuts down at the Viktualienmarkt and I want to surprise the wife with something out of the ordinary.
hi! there's lots of room for more info on this, but you may be interested in a couple of earlier posts