I wouldn't be surprised if I have to do some mod like that. I haven't heavily considered the Miata wheels due to the price.
TireRack is high on my list now, for sites that give you the info you need.
I was able to quickly find the 14" Kosei K1 TS. They only weigh 9.3lbs and cost $119/ea.
That's basically what I suspected I could find, price/size-wise (due to other findings in the 15" dept), but for the life of me, couldn't.