As X-Pilot noted, the correct size is 255/55/R18...
A quick glance at TR shows 80 tire models in that size, with several in the well under a $ Grand for a set of four.
And, if not a TR fan, try Discount Tire, OnLine tires, Sears, Costco, et al...
Not pitching any, but some sets that pop up include:
B stone Duellers
Yoko Avid
Gen Grabber
F Stone Desitnation
Kuhmo Ecsta
et al
Tire Search Results
GL, mD