Does the barrel have to be made by the Original Manufacturer to fit or can any barrel company make one to fit ?
I've made several barrels of various calibers for my H&A's. Two were from Green Mountain (link below), which are nice because they are already threaded for a 5/8" x 18 breech plug. At one time you could buy plugs from H&A. but those days are over. However, it's not difficult to fabricate a plug from a 5/8" x 18 steel bolt. One was a barrel blank of unknown manufacturer I bought at a gun show. I had to thread that one for the plug. If you're a bit handy with tools it's a fairly simple job to set up a new barrel for these sweet underhammers.
BLACK POWDER MUZZLE LOADER BARRELS - Green Mountain Rifle Barrel Co. (gmriflebarrel.com)