Try your luck at either Gun Parts Corporation (the old Numrich Arms Corp.) or at Sarco.
Either of these can be found in any edition of Shotgun News, and I am sure that there are Internet links, although I don't have any handy at this time.
If these don't provide what you need, it's time to start scrutinizing the shotgun news and gun list magazines for parts and/or donor guns.
You should get some help from the first two that I mentioned, although there are more surplus arms dealers out there. Century Arms is another, and I am sure that someone will chime in with others.
Good Luck to you.
Unkel Gilbey