There are still double barrels made in Italy. But the price is pretty high.
Single shot percussion and flint smoothbores can be had for about the same price.
There is a club here in York county that has a few black powder trap/skeet matches a year. (The Tomahawks)
There are still old doubles out there that can be purchased and restored to shooters for less than the cost of new.
A few companies offer kits to make single barrel smoothbores. A good quality black powder shot gun barrel is often as expensive as a good rifle barrel. They are heavier at the breech and narrower at the muzzle than rifle barrels, which means more cost in machining on the outside.
An inexpensive alternative is a smooth bore barrel for a rifle, or just having a guy like Bobby Hoyt ream a barrel smooth for you. I recently purchased a CVA Bobcat for $60 and paid a gun smith $40 to ream the rusty barrel to smooth. I still have some work to do to it, but it is a smooth bore barrel suitable for small game with shot, or for deer with a patched round ball. It isn't a large gauge but will work. Nearly any 50 caliber barrel can be reamed smooth to 28 ga (55 cal) or 24 ga. (58 cal)