I bought a 370z... but it didn't come with the right key.

MacCool

This story falls under the heading of caveat emptor. The entire hassle could have been avoided by addressing the problem about 2 1/2 years ago, or by doing what the Mitsu dealer suggested...taking it immediately to the Nissan dealer. Seems to me that a used car dealer probably would have a legal obligation to provide an appropriate key at the time of sale. 2 1/2 years later...? Unless that's a really stand-up dealer, my suspicion is the the OP is SOL.