You could always take your exacto knife to an old inner tube and make your own diaphragms.
Seriously, though, any NOS (that's New Old Stock, for anyone wondering) diaphragms will still be decades-old material.
The other issue in these niche-market parts, is that they would have to sell for enough to justify their small-scale manufacturing. That means your replacement diaphragm would cost far more than would a new valve.