You can buy one at your dealer, or through a "speed shop", and there are a ton of places online. Just remember that this is a "new" car, and it has warranty, if you have a rust issue while it is still covered they might give you a hassel (if you get on from the "Factory" and get the dealer mechanics to install, this might not hurt your warranty. Best of luck.

I would go to the lexus dealership. You dont wanna chance any after market accessories