Try gunpartscorp website. They have a cross reference to find the make and model of your Sears shotgun.
On line auctions, gun shops, gun shows, pawn shops.
Proofhouse.com has a store brand chart. When you find out who made it a value can be estimated.
The stock for this model gun can be found at cabella's
It a Savage model 30H
You will not likely find a Remington stock. I put my 722 in a Bell and Carlson stock and I love it.
http://www.e-gunparts.com/products.asp?chrMasterModel=0780z33&MC= this link is to Numrich gun parts. They have a listing of the 33 and show most parts. Hard to tell what they have in stock without calling them. Usually they are pretty good. iflya310 at yahoo The Model 33 was made by High Standard. You can find references to it in the High Standard Forum.
We need to know a Model 31 WHAT? If you are referring to a firearm, the serial number is NOT a sticker on the stock.
Try e-gunparts.com
there are several listed at gunbroker.com
Try e-gunparts.com
I have a used "riot" 520-30 butt stock.
A Marlin model 81 DL is the same rifle. You can put the same stock on your J.C. Higgins. Now to find a stock for a model 81. Try gunbroker.com or perhaps Numrich gun parts.