Try International Military Antiques (also at http://www.ima-usa.com and 908-903-1200 ph. #) in the Shotgun News. They sell reproductions of them for around 99.95, and include the bayonet mount. Their ordering code is MU1048. They are reproductions though.
You might also want to check in the military shotguns forum on http://www.gunandknife.com for a real or reproduction one.
