Discount Tire in Eden Prairie has a set of Falken 235/40/18 RT615 tires. The price seemed high, but they are in stock. $183ea
Ouch. Call Sears auto (any). that's where i bought my falkens from, and they beat discount so bad, discount wouldn't match them. even when i brought in the receipt.
Further more, DON'T BUY THOSE TIRES!!! they are the WRONG size. They are taller, and while this will give you a better top speed, it will SCREW your accelleration. Trust me, i was in this boat last year. i ran the wrong size tire and it really hurt my acceleration out of a corner. something you just can't afford when driving a laggy turbo car in an autox.
Little more research, unfortunately falken makes **** for tire sizes (wow.. big supprise there) and the only tire that would be an upgrade and come close to your factory size on an 18" wheel, is going to be a 225/40-17. if ya had a 17" wheel, you could get away with 235/40-17s, but a 235/40-18 is just too tall to be a good tire for autox. trust me. i made that mistake last year, and I'm not making it again. And after a breif check, no one makes anything good in a 235/35-18.