Try your local gunsmith, rummaging through the bins at a gun show to see if you can find a match.
www.gunstockinc.com
it is a Stevens, parts are interchangeable...
Have you tried e-gunparts.com?
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.
Start your search at Numrich gun parts corp.They can be found on the web.
Yellow pages category = GUNSMITHS.
www.e-gunparts.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
Numrich Arms should have them.
A FEW parts are available from gunpartscorp website.
I would try Numrich gun parts corp.They may be found on the web.
The 238 is similar to the 258. For parts, try gunpartscorp.com