Where can I buy a header for the rx8?

 

there aren't any that you want. check the links at the bottom of this page and learn a little more about why, and wait/make a unit which would be effective. as it stands, your run of the mill equal length "tuned" header isn't tuned for the way the 13BMSP's ports work on the exhaust side, so it would have little to no advantage over the stock unit.