There really isn't any "budget" competition tires. These tend to be serious tires and cost serious dollars. Right now would be a good time to be checking for used tires. Check local SCCA and NASA web sites. Many track guys will be selling this years tires as they don't want to store them. Most of the major tire makers have race tires in 15" diameter as well. Here's a few:
Kumho v710, BFG Gforce R-1 and R-1s, Hankook Ventue z214, Hoosier A6 and R6, Toyo Proxes R888, RA1 and Yoko Advan A048
All of these are available from the Tirerack.