I saw mutton tallow listed in Dixie Gun Works' on-line catalog earlier this year. A month later I tried to order some, but they were out (I just looked, and they don't have it listed right now, either). We tried to get some locally with little luck, the local meat processor didn't have any sheep or lambs just then. The censuses was there wouldn't be any lambs locally until after the county fair's livestock auction.
Eventually, we found a deli/meat market that advertises they do custom butchering (specializing in animals from the fair). After the fair, we got a few pounds of lamb suet. Then, we rendered the tallow.