I have not looked for this year's motors yet, but the following has worked in the past.
Fisher Price service centers should have them. You can get a list of local service centers from
http://service.mattel.com/us/powerwheels_info.asp. There are two problems, though.
Fisher Price service centers are not supposed to sell the motors or gearboxes to the general public. A few years ago, there was a memo from Fisher Price to their service centers saying that they were allowed to sell to FIRST Robotics teams. In my experience, everyone has either lost or forgotten that memo, so it may take a few tries to find one that will sell to you.
The next problem is that the service centers usually stock the motor and gearbox as an assembly and that PN doesn't match with anything that FIRST gives us. You'll need to find a service center that is willing to go in the back and manually verify the PNs printed on the motor. Last year, the service center didn't have any of the right model in stock, but did order them direct from Fisher Price and it only took a few days to get them.
Hope this helps.