If you want to stick with NIMH batteries, almost any decent battery, 3000MAH and above will work. You can find them at any hobby shop (in store and online), Amazon, eBay, Walmart, as well as many other places. To stay with the stock connectors, just look for a NIMH with 4MM banana connectors. If you want to go Lipo, as stated by @Tunedpipe, start by learning about them first, and then look for a 7.4V, 2S Lipo. The higher the MAH rating, whether for NIMH or Lipo, the longer the run time will be. For NIMH, i think the highest you can go is in the 5000MAH range, but, for Lipo, you can go as high as 7000-8000MAH. The first thing to do is check the max dimensions of the battery tray, and then check the specs on whatever battery, NIMH or Lipo, and, as long as it will fit the battery tray, it will work with it. I run ZEEE 7200MAH 2S Lipos in mine (disclaimer: i have only had mine about two weeks, but, i went with Lipo right from the start. I never use NIMH batteries anymore).
