I bought 2 hobbyking nano-tech 850mah 2s packs ($5.99 each) and made a parallel saddle pack. Slightly too large for use with the battery posts on the gt14b so I bought some small Velcro straps which are held using a nut and bolt through the post holes. Some Velcro on the chassis and on the bottom of the batteries raises them up to stop any rubbing on the nuts.
To charge and balance the pack I bought 2 balance leads and made a Y connector to attach to the 2 batteries. If you're worried about this homebrew setup it's basically exactly the same as a large capacity 2s lipo if you took it apart and examined the cells and wiring.
Link to batteries here
http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html