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
it is a Stevens, parts are interchangeable...
Have you tried e-gunparts.com?
Buy parts for 410 Stevens model 67 pump shot gun.
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. firstname.lastname@example.org
Start your search at Numrich gun parts corp.They can be found on the web.
Try Numrich gun parts corp.or gun broker.com under shotgun parts.
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 too have a Ranger .410 sxs hamerless shotgun. I believe it to be made by Stevens. Possibly a model 311.
I can say that the Stevens model 311 side by side shotgun was discontinued in 1989.I would go to the Savage arms web site,they may be able to help you find out when the manufacture of your shotgun was started.
If it is side-by-side, probably Stevens.
Yellow pages category = GUNSMITHS.
There are some sources for Crescent shotgun parts, but it is very possible that parts will have to be repaired/made.
The model 5100 Stevens shotgun has a plastic stock