I ran into this exact same problem with an old dodge dart I had. Found that noone made them anymore. If you cannot find one anymore from anyone, you need to go junkyard hunting. Junkyards usually pull the gas tanks from cars b4 the cars go out, but 9 out of 10 times, the donut sticks to the filler pipe, and is there for the grabbing without any tools!!
Mine went bad awhile back and we made one out of one for a later year car. So far it has worked like a charm. Good luck