2005 miata transponder key
 

I want to purchase a couple of spare keys w/o remotes for my 2005 Miata. Dealer prices are outrageous. Found reasonably priced blanks on line but because dealer stated that he'd need VIN to order correct blank, I'm reluctant to order on line. Can anyone tell me if the blanks are so specific that they must be referenced to a VIN ? Sounds fishy to me.