i got mine (54 3600) through my local auto parts store. $19. i dont know if this will help you much but it was an ATI (auto value) store and the part # was 411330n.
have you got the pinion nut off? if not, have fun. if you do, you can use a micrometer to measure the od and id. that will help the auto store folks get you a seal that will work.
there is also a piece of felt rope stuffed in front of the seal. mine was in good shape and did not need replacing but if your's needs to be replaced that was a part i could not find. you might be able to use water pump packing.
a speedy sleeve may be needed as well if the pinion shaft has been worn by the old seal.
