considering I don't think there are any RX-8s even on the road yet, the Mazda dealer would be the only place to get one. I would imagine that it would fit. I believe the Miata and 6s knobs fit our cars A OK
It's not big deal, but the RX-8's shift knob has the numbers 1-6 in the knob. I think it would be kinda silly to put it in a car that is only a 5 speed. Not to mention the knob is rotary shaped.
Well if the RX-8 has the same thread pattern as the RX-7, then the shifter will fit. I used to have a second gen leather wrapped knob on my pro until I got the leather Mazdaspeed knob.