I have a bunch of nice old pumps, some of them T, A, and V8 Ford, that have tired washers. When I went hunting, I found very little out there until Googling around kicked up a pump resto thread on a British sports car group...look at this:
http://www.base-camp.co.uk/Cups%20.htm
I have NOT yet tried them, but I plan to measure all my pumps and order a whole bunch when I do.
If they fail me...I think the next step would have to be a hole in a block of wood, a piece of leather hacked out of a boot, and a central plug to force something to shape.