I have made antique outboard motor fuel pump diaphragms out of rubberized gasket material. Check with any industrial rubber and gasket supplier, and they should be able to point you in the right direction. I've been given scraps of thin rubber gasket material big enough to make dozens of small diaphragms from these places.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!