You can try e-gunparts.com, local gun shops, local gun shows, want ads.
Numrich Arms should have them.
A FEW parts are available from gunpartscorp website.
The 238 is similar to the 258. For parts, try gunpartscorp.com
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
it is a Stevens, parts are interchangeable...
A breakdown of the parts for a Stevens Savage Arms Model 9478 is found in the book The Gun Digest Book of Firearms Assembly/Disassembly Part V - Shotgun. It has exploded views of the stock and its availability on the market.
No Model 772. How exactly is the shotgun marked? sales@countrygunsmith.net
savage arms
1930s
The exact value of a Savage Arms Stevens model 77m shotgun is actually dependent upon a number of factors. Some of these factors would be the age and condition of the shotgun.
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
Numrich Arms has parts for the 311