Do you want to keep it original or just get a reliable switch?

If the switch assembly is worn, and you want to preserve your originality, I'd find a good used replacement - available from several of the individual parts suppliers on the board - Chris Horn and Jim Morris are both excellent guys for such things. I've kept a couple spare sets on hand as well if you have no luck.

If you are talking about the tumbler and key, I think you are also better off going with used. These are significantly easier to replace.

I think Ron Kwas might recommend a hidden starter switch to solve your problem.