Sprocket gasket material?

[Cycofast]

I'm homebrewing with a gasket I bought from a gokart supply company. I need to find some of the rubber gasket material so I can make a custom gasket for my wheel/sprocket mount. Does anyone know where to purchase this stuff? I'm not sure if it's rubber...it might be fiber based but I'm not sure. I'd appreciate any suggestions.

 

[Norman]

the sprocket gasket /spacer/ fiber material is sidewalls from auto tires.

 

[Bikeguy Joe]

The last kit(s) I got there were some made from very worn TREAD!