I bought a whole bunch of 1/3 AAA off of ebay a long time ago. The electronics can handle 4 cells. You could take two batteries from another truck. The 1/3 AAA I have are slightly bigger than the ones in the truck.

You can still get them from Ebay, I'm not sure where else. I bought 150 for $25 back in the day, so I haven't had to worry about getting more since.

I also added a dpdt switch and wired it up to switch between parallel and series connection between 2 packs. This way I can still charge it on the base controller, only I charge 3 times. Then I flip the switch to run at 4.8V and she flies. I added a 9 tooth pinion to make it faster too. That truck is a lot of fun.