Numrich arms has a good supply of new 870 stocks.
The Stevens Model 77E was a pump-action shotgun offered in 12 gauge, 16 gauge, 20 gauge, 28 gauge, and 410 gauge. The military version 77E was the most widely used shotgun of the Vietnam War. It was a short-barreled pump-action shotgun known variously as the "trench" or "riot" shotgun in 12 gauge. The Military Model 77E had a noticeably shorter stocks than similar United States military shotguns built by Ithaca Gun Company, Remington Arms, and Winchester Repeating Arms Company. These short stocks were intended to accommodate South Vietnamese soldiers, and the Military Model 77E was the first United States combat shotgun equipped with a rubber recoil pad. Military Model 77E shotguns were Parkerized with sling swivels and wooden stocks. Receivers were marked "U.S." and "p" proofmarks appeared on both barrels and receivers
Try the maker.
Wooden dowel
The stock...some have two piece wooden stocks - called fore stock and butt stock.
Gun shops, gun shows.
"Stocks" or "pillory."
Wooden stocks (the part that you put into the pocket of your shoulder when you're in a firing position) are made from trees. The American military as well as most European militaries don't use wooden stocks with their weapons anymore. It's mostly plastic and polymer. Russia still uses the AK-47, which typically has a wooden stock.
Yes
Stocks or pillories were wooden frames used to publicly humiliate and punish lawbreakers by restraining their hands, feet, or head as a form of physical punishment or public shame.
Gun shops, gun shows, want ads.
http://www.e-gunparts.com/productschem.asp?chrMasterModel=0840z500
They were used to punish lawbreakers. Stocks were a wooden device in which a person's arms and head and legs were held down in an awkward, uncomfortable position. The person would be displayed before the entire town in the stocks, so not only the physical discomfort would be a deterrent for further wrongdoing, but also the public mockery and embarrassment.