You can buy the gearboxes from a Power Wheels dealer or repair center. I just bought eight spares for three different teams from one in Michigan. Here is the link to the PW web site:
http://www.fisher-price.com/us/power...p?type=service
The gearbox w/motor has a part number of 00968-2902. The gearbox without motor should be 00968-2802. The gearbox with motor is about 30+tax.