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 email@example.com .
I would start my search at numrich gun parts corp.They can be found on the web.
it is a Stevens, parts are interchangeable...
Have you tried e-gunparts.com?