From McMaster-Carr you can get nitrile (NBR, Buna-N, etc) in 1/64″ which is about 15 thou. I have left this stuff soaking in gasoline for months without much obvious deterioration. They also carry Viton sheet in the same thickness. I’ve not tried that–it’s harder and could be too hard for fuel pump diaphragms.
https://www.mcmaster.com/materials/thickness~1-64/
