Try: http://store.summitracing.com/product.asp?p=524&searchtype=ecat
Part # EDL-5509 3" inlet two 2.5" outlets 147.69 for the muffler.

I have an Edelbrock RPM EDL-5501 on my CSP Miata and it is very free flowing.

I don't know if the EDL-5509 would be quiet enough for the RX-8 though.

I'll measure it up and if it fits, I'll make one up (down, dirty and ugly) and see if it works.

Vince

The inlet on the stock muffler (and the cat-back pipe) is 2.5"
You will have to do a little fitting to get the pipe to fit the 3" inlet.

Let us know how it works out.