I would contact Savage Arms (they own Springfield) and ask. Customer service at their website would be a good start.
www.gunstockinc.com
Try e-gunparts.com
The Stevens 311 is a double barrel shotgun.
The model number is stamped on the right side of the receiver on many Stevens products.
Yellow pages category = GUNSMITHS.
e-gunparts.com
gun-parts.com has them...but for 12 ga only
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
Probably at Savage customer service website (Savage bought Stevens in 1920)
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.
Numrich Arms should have them.
There's a whole shelf of books on firearms in your library. Or you may find information on www.savagearms.com