Muzzleloading is sort of a niche in the shooting world and muzzleloading fowlers or shotguns is definitely a niche within a niche.
Because no area that I know of has a special muzzleloading only waterfowl or migratory bird season, muzzleloading shotguns have escaped the inline-azion of muzzleloading. You don't get an extra hunting season because you have a muzzleloading shotgun so they are bought by people who love the lore of old muzzleloaders, not by people who buy a muzzleloader as a means to an extra hunting season.
Because of this, you are not likely to find many muzzleloading shotguns in pawn shops.
Visit the Nationals in Friendship IN or visit some other muzzleloading shotgun trap/skeet shoots and you'll come in contact with guns for sale and meet builders of fowling pieces and learn more about these guns than you'll ever pick up off the internet.