Jim, it isn't rocket science to make one. a compatible breech plug and wedge tennon is available from TOW. Breech the barrel, cut dovetails for the tennon and sights and silver solder or epoxy the under rib in place. I have a gun scrounged from TC, Lyman and Investarms parts. I fitted a few barrels to it. 45, 50 and 54 cal. barrels.
Or, buy an abused barrel on line and have it lined for 36. That is something Bobby Hoyt in Fairfield pa can do.
I just got an abused 50 cal barrel for a CVA bobcat for $25 (included shipping) which I am going to have reamed smooth to use as a shotgun. If I find another cheap, it will be lined for a smaller cal.
In fact on the big anti gun auction sight, a lot of three TC abused barrels with tennons, etc just sold for about $55 plus shipping.
It may not be as convenient as a ready made drop in, but where there is a will there is a way.