You probably cannot buy them anywhere, since it does not appear on the IFI FIRST parts store page
http://www.ifirobotics.com/first-store.shtml
But it is likely that there will be teams bringing their unused motors to the regional competitions, so you might be able to scrounge one if needed.
Also you can post a wanted ad on the CD-swap, maybe a team who's not using the motor will give/sell you theirs.