You are not going to find a cantilelver smooth bore barrel new, I can almost assure you of that. The only cantilever mount barrels you will find will be fully rifled. And you can get them for a remington from remington, mossberg and hastings.

If you want a smooth bore with a scope mount I think you are going to have to look for a used one, if they even ever made one. I have seen them with open sights, but not with a scope mount. Doesn't mean they don't exist, I just have personally never seen one.

Paul