If youre looking to go fast for a short time hoosier r6 is a great track tire. If you want something that will last and still be fast look at the nitto nt01 or kumho xs as stated, the nt01 can be bought 235-40-18 and is a great track or autocross tire and ive found to be as fast in autocross as the hoosiers, the kumho is another tire that can eb found in our size on ebay 285 35 19 2 rears for about 170 a piece and front 235 40 18 for 75 a peice. Also another one is the hankook rs3 great tire, loves the heat and decently priced.
Hankook also just put out a new one ive seen on ebay a 80 treadwear i belive, the z221, supposed to be a great track tire.
But you really need to decide if you want to go as fast as possible for a lesser amount of laps or a little slower and more laps.