I would first start my search at numrich gun parts corp.they can be found on the web.You may also look at gunbroker.com,or guns america.
The model is obsolete, however we have a good supply of parts for that model. sales@countrygunsmith.net
Numrich gun parts
I would start my search at Numrich Gun Parts Corp.
Try Numrich Arms, they've got a website and a lot of used parts.
No
I would try either Numrich gun parts corp.or look at gunbroker.com.under gun parts.
Winchester
DON'T!!! unless you absolutely have to. you can cause damage if you don't know what you are doing. and parts are hard to find.
Try: Numrich gunpartscorp.com Jack First jackfirstgun.com Bob's Gun Shop gun-parts.com
According to the cross reference in the Numrich gun parts catalog the Sears model 300 is really a Winchester model 1400.
The Winchester Model 21 is a side by side shotun, not a pump.
Have you tried winchester,s own web site first?if you had no luck,maybe numrich gun parts corp.may have one,or you might find one on E-bay.