Big Al1, if you would click that link to The Rifle Shoppe in my post above, where it says "British Wall Gun," it will take you to a web page that shows all of the parts needed to build a replica of a British wall gun, including the barrel. The Rifle Shoppe will sell the parts individually or as a complete set. I think they charge $495.00 for the barrel alone. Sights and under lugs are extra. If you get the complete set with the lock assembled, it will cost you somewhere north of $2,000.00. Then you need to build it, or hire somebody to build it for you.
Good luck!
Notchy Bob