Try your local gunsmith. The Riverside is a Stevens shotgun. If you can find out which model it is, you can probably find the wood at Numrich Gunparts
Riverside was manufactured by Stevens, so if you can identify the equivalent Stevens model, you should be able to find parts. Check with a local gunsmith.
Stevens/Savage used the Riverside name from about 1920 to 1945.
Can't give you an exact date, but Stevens used the Riverside name from about 1920-1945 according to my notes.
try looking at savage or Stevens shotgun diagrams. I found a firing pin for a 16 gauge riverside it was an old model 94 Stevens part
Riverside Arms was a tradename used by Stevens and later by Savage. Would help to know what other markings are on the gun.
Stevens made the sb-300. You can cross reference it and it comes out to a Stevens 311
Basically a Stevens "store-brand" shotgun from the 1920's. firstname.lastname@example.org
The model 5100 Stevens shotgun has a plastic stock
Try Numrich Arms (www.e-gunparts.com). Look for parts for a Stevens 311 shotgun. They should be the same.
Whats the valve of a Stevens model95 12ga shotgun
Riverside was manufactured by Stevens, so if you can identify the equivalent Stevens model, you should be able to find hammers at http://www.e-gunparts.com/ Or if you still have one hammer (or a broken one) you can take it to a local gunshow and dig through bins of miscellaneous parts looking for a match.