Good luck finding anything even remotely close to those sizes. 14x7/7.5 is probably going to be a custom size.
Did you buy the tires used? If not, just curious why you chose a staggered setup, since Miatas tend to prefer same tire size at all four corners. In other words, just 'cause your SHO understeers doesn't mean you have to inflict the same fate on the Miata.