NAD: I bought an 18.5" Benelli M1S90 factory barrel a few months back. I ordered it through my local gun store, which is an authorized Benelli dealer. It took three months to get it, and my dealer said that Benelli is famous for slow service when ordering replacement parts. It wasn't cheap, either, but that's the only source I know for replacement barrels.
Good Shooting, CoyDog