First try the local gunsmiths. Check at a few gunshows if there are any in your neighborhood. You might try Numrich Gunparts.
Numrich Arms has parts for the 311
there is the Stevens side by side shotgun model 311
www.gunstockinc.com
it is a Stevens, parts are interchangeable...
Stevens
Have you tried e-gunparts.com?
Buy parts for 410 Stevens model 67 pump shot gun.
Jack First Gun Parts. Search the web, they have a web site with the phone number. They do not have a catalog on line.
I don't find the Westernfield 530A but be careful: there is a Stevens 530 (1912 to 1918) that is a pump shotgun as well as the Stevens mod 530 (1935 to 1952) double barrel.
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.
Check on Numrich gun parts corp website.
I have parts for that model. sales@countrygunsmith.net