They show up on kingsnake's classifieds occasionally, there are a few breeders out there. They're not common in the trade though, so expect to pay quite a bit more than what you would for the more common species. They are very cool though, probably my favorite musk turtle.
I have the the loggerhead subspecies, Kinosternon minor peltifer if you are interested. They are a lot harder find than the loggerheads. Pm me if you are interested.