Have you checked www.tirerack.com
yet? I would stick with a 15x7" wheel and run the widest tire I could fit on the wheel (probably a 225 or 215 section width). You'll want to opt for a slightly lower profile than normal if these aren't going to be your street tires simply due to the quicker response a smaller sidewall will give you.
Are you new to autocross? I would just buy some high performance or max performance summer tires for your stock rims and just work with that for now. The wheels aren't going to make you faster, and the practice behind the wheel is the most important factor in getting quicker.