brake reservoir cap rubber

can someone point me in the right direction as to where to find the rubber gasket that go under the master cylinder cap on a 73-76 Duster and Dart Sport. for some reason both the ones on both cars are leaking. I tried to buy one back a couple of years ago for the Dart and it didn't fit. I think it was a Dorman, not sure. I can post pics if needed of these.