Wow! You guys are really up against it with those SBE shotguns.

I did some research on the barrel availbility and the barrels are out there....both on availability and price.
You might want to consider the purchase of a Benelli Nova (new or used) and purchase a cantalever rifled slug barrel for it from Cabelas for $149. The cantalever barrels are actually made by Ithaca. Ithaca's web site lists them for $199.
I'm guessing you could purchase a used Nova for $200 and a new replacement barrel for $149 and have less or nearly the same money invested as you would purchasing ONLY a barrel for your SBE. Plus, you'd have a darn good back-up pump shotgun.
Good luck with your choices!
Good hunting, Bowhunter57