I have used various sources for such rubber type materiael ---golf cart floor mat(old ones with hole by gas pedal have lots of other good areas), truck mud flaps from along side of road, one time I was in supermarket whan they were changing checkout belt, gave me old one with torn edge, while on vacation in China I went to the buisness materiael district, found a rubber store, they had everything, found some rubber in the same pattern as my 53 Citroen needed --for 10$ I got enough in 3 thicknesses to have 10 attemps at making what I need---then to the sandpaper store, then ball bearing store ---the same flapper sanding discs that are here at 5-9$ were !$, ---
The old time plumbing store also has had some rubber sheet gasket in different thicknesses that I have used for body shims and as has been mentioned, Mc master carr, grainagers, Fastenall, reid supply etc.