I'm going today to my old job (auto upholstery shop) to order materials for mine. Planning on changing my tan oem leather out for black vinyl sides back and bolsters with black suede pleated inserts, all done with green thread to compliment the exterior color of the car.
You can not however get new seat foams for a miata, at least NA and NB because they are out of production and no aftermarket company makes them. What we end up doing is sectioning out the old foam, gluing in a new block of foam and sanding it down to match the original shape. My seats are in fairly decent condition though so what i will be doing is steamimg the foam to bring some of its softness and shape back, then building up the bolsters with more foam to make it a bit more like a race seat bolster. This requires that you modify the seat cover design, but i have done it a thousand times so it wont be a problem for me.
Also, another bit of information for someone wanting to try this, miata seat covers are different from the passenger side to the driver side. If you want a perfect fit, it will require that you take both sides apart for patterns. You can use just one cover for a pattern, bit you will never get the other seat to turn out perfect that way.
It will be a while but i will post up pics in a new thread when i am done.