Check on some Enkei Cast wheels.
Personally, i would try to run some larger wheels for a 2nd gen Rx7. 15 x 7.5 and 15 x 8 seem to be sizes more common with miatas (reason why most wheel manufacturers are only making these in 4 lug)
I am sure the 2nd gen Rx7 should run a lot more than that especially with a 5 lug.
There are plenty of nice 16 and 17 inch BBS RS Forged 3 piece wheels that will fit nicely with a 5 lug FC as well and are around 800 ~ or so for a set of 4.
Check
www.Rbwheels.com for used wheels