I would try Numrich gun parts corp.They may be found on the web.
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.
Yellow pages category = GUNSMITHS.
Numrich Arms should have them.
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
A FEW parts are available from gunpartscorp website.
The 238 is similar to the 258. For parts, try gunpartscorp.com
I am not sure about the diagram,but I would check at numrich gun parts corp. first for the parts you need.
You might find what you need at firstname.lastname@example.org .
I would start my search at numrich gun parts corp.They can be found on the web.
Contact Customer Service for Savage Firearms. They have owned the Stevens name since the 1920's.
it is a Stevens, parts are interchangeable...