As has already been said, they are sometimes seen on eBay. If you go that route though, just make sure that the frame is the same as yours . . . i.e. don't try to put a ASM or a Pietta on a Uberti, etc. as it probably won't be the same.
Doc Hoy would be a good one to ask as far as interchangeable frames, etc. as he has worked on a lot of different brands. I'm sure others have as well.
If you know what brand, model, etc. you are looking for, I'd post a WTB (want to buy) on the classifieds here as well as over at CAS City - you never know what someone might have laying around.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63