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.
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.
Try e-gunparts.com
I have parts for that model. sales@countrygunsmith.net
Check on Numrich gun parts corp website.
Start your search at Numrich gun parts corp.They can be found on the web.