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.
About 100 US dollars
The same, Its a Stevens model 95, a Savage 96 and a sears 101.100
Depends on the model. We usually have parts.
$80-$100.
www.e-gunparts.com
50-150 USD
Marlin Model 59, 1959-1961