Man, that is the problem with old bikes - especially good ones. They don't have much re-sale value, but try to find a part for one! It will cost you a fortune IF you can find the part.

How about contacting the shop where you bought your bike. Any chance they are still around? I found that many of the bike shop owners of the '70's are now retiring, but they still have stuff in their basement, garage, or otherwise warehoused from their old bike shop. If you know the old owners, try contacting them.