Cam,
IIRC, the littlebox functions best at 9v. Jon will correct me if I am wrong.
RC batts are the way to go IMO, and they are easy to come by. The higher the capacity, the longer the run time.
Keep some 9v on the side ALWAYS just in case, but as willdmb says, the littlebox will drain the higher voltage first if you want to keep a battery in the tray.
There are also the options of 1) asking our resident cable maker Ted to fabricate you a specific DIN-3 cable to connect to your battery of choice, or 2) hacking the Tamiya connector and making your own.
Other than that I have mistakenly run it at 5v with no adverse affects, so you good a nice peiece of gear coming your way.