You might look at Auction Arms and Guns America. Most of the Muzzle Loading forums have classifieds sometime s you see them listed there. Sidelocks will be harder to find than an in line if you want a production gun. Look for the White Rifles Tominator and Thunder, both are great shotguns straight rifled barrels and interchangeable chokes. I own two Tominator's, got rid of my moderns cause the Muzzle loaders pattern better.
Good luck