You can order battery packs reasonably low-cost from many sources: www.cheapbatterypacks.com, www.batteriesamerica.com, www.radicalrc.com, etc.

all you need is a 9.6 v pack for the transmitter that will physically fit into the space of the old pack. And you need a 4 or 5 cell pack for the receiver. anything over 600 mAh is fine. If you know how to solder, you can make your own packs from wal-mart rechargeable nImH cells. But this is risky if your soldering is not good becasue you can ruin the cells or create a connection that fails under vibration.