I buy almost all my parts from http://www.irontoad.com The parts are all OEM and actually come from a Lexus dealer, they're all about 20% off though so it's a pretty good deal. The part you need probably won't be listed on the site, you will need to request a quote. They will email you back with a price for the part very quickly. Highly recommended.
After installing the new sensor you will probably have to reset the ECU....disconnect the negative lead on the battery for 10 seconds or more. If the light comes on again this will probably indicate that there is another problem you need to check out. I would first buy and install the missing part and go from there, let us know :)