I believe the paint code is on a plate inside one of the doors (I want to say passenger, but it may be the driver's side). In terms of where to buy it, once armed with the paint code, I just ordered some "Genuine Subaru" touch-up paint off of Amazon.
Edit: Whoops, I need to learn how to read. As others below pointed out, I missed the "wheel" part of the subject. This is for the body.