They come with a Deans connector, so you'll want a battery with a Deans connector. Use Google images to see the difference between Deans and Mini-Tamiya (which is the standard connector).
You'll want a reasonably small 7.4v LiPo as even though they have crane stocks the mosfet inside takes up a fair amount of space. You'll want a nunchuck style battery that isn't long.
Don't cheap out on batteries or battery chargers. Avoid NUPROL shit like the plague (except gas and BBs) .