Hate to tell you, but the the best source is probably s2ki.com. Lots of noise and JDM luv but decent info in the "racing and competition" section from most of the nationally competitive s2k drivers.
Most of the reputable companies like Sparco, Recaro, OMP, Sabelt, can probably tell you what fits.
For autocross, the generic seats at Pep Boys are probably fine. If you track or race you'll want to look for FIA certification.
Here's a vendor that sells to the s2k crowd, that also has a decent selection of seats. http://www.evasivemotorsports.com/m...Code=SEATS