This shotgun was probably made by Crescent, so your local gunsmith can probably find one in his parts pile. Or, if there are no missing pieces, he can repair yours.
Check with your local gunsmith. No one would be making new parts, but lots of the old double and single barrel shotguns were similar and he might have something laying around that he can make fit.
Sears 200 series = Winchester 1200. Parts should be available from e-gunparts.com if you can't find them locally.
Typical Crescent single-barrel shotgun. I have a good supply of parts. sales@countrygunsmith.net
My cross reference list indicates that this is a Savage 59. If your local gunsmith can't find the parts, you can check with www.e-gunparts.com
Parts for these shotguns are hard to find. EAA Corp carries parts for Baikal shotguns. A good gunsmith might also be able to help make some parts.
gunpartscorp.com
Some of the larger used parts dealers may have a small quantity. Try www.e-gunparts.com or Jack First Gun Shop.
Go to"krieghoff.com".
try: gun-parts.com
You can't... you will have to make them yourself or have a machine shop make the parts
You could try Gun Parts Corporation.
These single-barrel Crescent shotguns range from < $50 as a parts gun, to maybe $100 if in excellent shape.