I've had great luck on ebay. Heck I even bought my Lexus there.
http://cgi.ebay.com/ebaymotors/74214-R-18-06-07-08-LEXUS-IS350-FACTORY-OE-WHEEL-RIM-_W0QQcmdZViewItemQQhashZitem41561f72d0QQitemZ280617775824QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories