285/40/18 is teh size you should be getting, its much closer to stock size, actually its the size the gt500's run in the rear. i have this size bfg kdw's on my car, and they look great, good amount of sidewall and i usually like the lower profile look( stepped down from 20's to the 18's)
i dont think you will find a 285/45 anywhere with the exception that walter mentioned