Gas cap gasket

My friend has an M38 and an M151 and says that splash back from a full tank of gas with ethanol has turned the old gaskets to mush. Anyone know of a source for an upgraded gasket that is ethanol proof? A cork one would work but might dry out rapidly due to infrequent use possibly?
Thanks,
BC

Thank you. We will see if we can locate some.
BC

go with the cork one

mine is about 4 years old and still looks and feels new