and of course: tirerack.com
if you're doing open track events and not competing I would recommend you not go too crazy with semi-slicks etc. You'll improve quicker on a tyre that is more progressive at the limit, rather than a proper race tyre where the grip is more on/off. That way you can have a better feel for how the car reacts to different inputs. On race rubber its easy to make a bunch of small mistakes and not even be fully aware of it because the grip level is so high.