Are you talking about the turn signal cam (cancels the turn signal after completing the turn)? I need one for my 66 american also but have found that all of the ones in the junk yards around here are also broken. Have not checked parts stores for availability but someone always seems to have at least one available on ebay. Good luck.
It's a materials issue -- it's safe to say NONE of the used ones will ever be any good. In fact, nearly no NOS ones will be any good. The particular nylon used self-degrades with time, plain and simple.
You'll need to find recently-manufactured ones, which is of course nearly impossible; occasional old-time parts stores might have bagged replacements made on the 1970's, but that's about it.
There are some parts that are practically-speaking impossible to find. Some parts you can fabricate, but that canceller would be tough. It might be easier to re-engineer the problem, or find a switch assembly that can be modified to fit.