That I don't know grump...just seems like all the pics I see with 18" + wheels, there is NO tire around them, just all wheel.
Are you looking for a 30" tall tire out back? 29"? 28"? 27"? I'm sure you know, the taller the tire, the more "meaty" they'll look.
Still thinking about a staggered wheel size? 10" in rear...8" up front?
I suggest playing with these calculators, they will at least point you towards what sizes/brands you should be looking for.
http://www.rimsntires.com/rt_specs.jsp
http://www.miata.net/garage/tirecalc.html
Wish I could be more help. I think this is Derek's specialty, maybe he'll add some input.