I need to make a Doggie Door from ~1/4 " thick sheet vinyl, hinges, etc.

Where might I find a small quantity (2-4 ' sq.) of sheet vinyl?
Ran a search on 2 local big boxes and got diddly-squat. I am
in Saint Louis, MO.

Thx,
Puddin'

"Well, there's two trains runnin'.
Ain't neither one goin' my way.
One run at midnight,
the other run just before day."
- from "Still A Fool", Muddy Waters, maybe 1949