Well...that's a hard question to answer. I would go for something like this: http://www.hobbyking.com/hobbycity/store/__8933__Turnigy_2200mAh_2S_20C_Lipo_Pack.html
These are RC hobby batteries, so they are designed to work with high current draw motors. Problem is that they are 7.4V, which MIGHT eventually blow up your motors. Most DCs can run above their rated voltage (they just run faster) but you shorten their life. Your motor controller can take the voltage.
Another option is to just get a whole bunch of 1.2V recharge-able batteries. My experience has been that these do not last quite as long as the RC hobby batteries, but they are far more standard.