I suggest you google "rubber supply" company and look for a rubber supply shop near you. Here in California we have a few industrial supply companies near by and they stock all kinds of rubber goods. I have found them very good in supplying various rubber parts that can be used for gaskets, shims, mats, spacers, etc. The trick is to find a counter man that is willing to talk with you and show you what goods he has on the shelves in back. You may have to purchase much more stock than you need, or sometimes they have remnants. Good luck.
Bert