Any of the 124 series.
The model 5100 Stevens shotgun has a plastic stock
Try e-gunparts.com
gun-parts.com has them...but for 12 ga only
Try searching for a Savage model 620A or Stevens 620 stock on ebay, or gunbroker.com. The model M060 Westernfield is a Stevens 620, while a Westernfield model 60 is the Savage 620A
http://www.gun-parts.com/index.html/
Try e-gunparts.com
model 94B Stevens plastic stock was mfg in 1950. Stevens uded letter code starting with A in 1949 until P in 1969.
A breakdown of the parts for a Stevens Savage Arms Model 9478 is found in the book The Gun Digest Book of Firearms Assembly/Disassembly Part V - Shotgun. It has exploded views of the stock and its availability on the market.
Bobs gun shop google it! $85 + 95 stock forensic $40+ 35
I would first look at numrich parts corp,and look at the model number you have.If you do not find anything The shotgun you have was made by stevens and there model number was stevens model 520.I would look for that model number next.Good luck in your search.
If the owner's manual doesn't have instructions, seek out a qualified gun smith.
20,000 240's between 1942 and 1945. Originally had a tenite stock and forearm