Easiest place to find a replacement bbl. is E-Bay.If you get just a bbl. it will reguire some fitting to be done by a smith that knows Md.12's. If you get a complete front end asy., or a bbl and receiver extension, it may fit. But odds are it will need a little work also.
The best place to have Md.12 work done (at a affordable price) is Wright's Gunsmithing in Pickneyville Ill. Contact them for price info and any information that you may need.