Hmm, actually I found some 18's in the list, added them, clicked the TPMS link (which now said only $98 (!) for the sensors) and got this message:
"Your vehicle is equipped with a tire pressure monitoring system that requires a sensor to be installed in the tire and wheel assembly for proper operation.
At this time, Tire Rack does not have sensors available for your vehicle. The original sensors from your original wheels can be used or you may purchase new sensors elsewhere."
