I hope the tank battery is NiMh/NiCd not LiPo? If it's the latter then you need to be careful as they require a low voltage cutoff to stop them being damaged from over discharge. If it will tolerate up to 12.6V then a 11.1v (3 cells in series) lithium battery would be ideal since when fully discharged it would still be around 10V. Plenty on HobbyKing RC Plane, Car, Heli, Boat and MultiRotor Hobby Store ... if it's the first time you've seen that site you'll no doubt be up all night buying better batteries and all sorts for the tank!