Gun shop, gun show, want ads.
Try gunpartscorp.com
Sorry, but you won't.
You might find one at Numrich Gunparts
I would start at Numrich Gun Parts Corp.
able firearms on the web
You might find what you need at webdept@gunpartscorp.com .
Your Stevens model 77C was made from 1955-1971.
Measure it, or have a gunsmith measure it, with a choke gauge.
500
Your stevens model 9478 was made by savage/stevens between the years 1978-1985.
i have a model 200 20 gauge and i belive it was made in 1918
Savage Stevens