Are you looking to purchase old military guns? If so, gunbroker has about anything you could think of.
I saw some old Winchester bomb/arsenal marked shotguns a while back. What made them even more unique is they had pheasant hunting scenes engraved on the side of the receivers too. But, they did have a letter of authenticity from Winchester with the serial #'s that they were sold to the military.
I would personally like to have an old Ithaca model 37 or three.
I think Classic Firearms had some Ohio State Patrol engraved Remington 870's a while back. And some California Fish and Wildlife shotguns a while back.
I'm sure there would probably be a ton of prison/dept of correction shotguns out there too.