Depends on model and condition. It was not made by Sears, but by another gunmaker FOR Sears.
High Standard
Probably Harrington & Richardson
Your shotgun was made for Sears by Stevens, and is their model 311. It should take 2.5 inch and 3 inch .410 shells.
You can find a store brand chart at e-guparts.com that might help.
The same, Its a Stevens model 95, a Savage 96 and a sears 101.100
About 100 US dollars
Ted Williams shotguns were made by Ithaca and sold thru Sears. Some of this model were made by Bolito of Brazil and will have "Brazil" stamped on the receiver.
Depends on the model. We usually have parts.
$80-$100.
www.e-gunparts.com
50-150 USD