Some generic advice (sorry if it sounds like a broken record):
Before you buy anything, it is suggested that you read the tech guide (
http://www.thscc.com/timetrial/nonhtml/ ... _Final.pdf) to make sure you have got the right "master plan" in mind. If you have questions, ask a member of the track team ahead of time - we are here to help.
Make sure the seats you are getting (remember, equal protection for your instructor) are either FIA approved (and don't require a back brace) or that you have appropriately mounted seat-back braces and that your harnesses are mounted per the GCR and the manufacturer's recommendations.
You should also try and sit in some seats before buying - they are usually made in different widths. As to fitment in your car - other owners are a good resource but be careful who you listen to. If you find a model or two that are used consistently and fit well in the information archives for your vehicle, but are no longer manufactured, you can probably find the dimensions of the seat to find something similarly sized that is still in production.
HTH,
--Ashraf