There are alot of Lithium Cell Phone batteries out there that will fit the existing battery compartment.
One big advantage to the Cell Phone batteries is the fact that they have a pcb in the case. It will monitor the charging/discharging process. It will shut off the juice if that cell goes below 3.0 volts per cell. When it's plugged into the charger, the switch resets itself again.
If the cell is attempted to be overcharged, it will shut off the power to the cell at around 4.3 volts per cell. It's a built in safety.
Here's an example.
http://cgi.ebay.com/T-E-maxx-Traxxas...QQcmdZViewItem