Bought mine @ HobbyKing,
http://www.hobbyking.com/hobbyking/s...ddle_Pack.html
so far they are holding up, the rating is only 4800Mah at the time i bought mine but i believe they higher one are also available. Shipping is a bummer, it took almost a month for item to arrive at my doorstep ( due to Chinese New Year) but if your not in a hurry then they are ok for the price.
Gen Ace batt are alsook ( well read some are not thought), but if you want to be sure search google for feedbacks on these batteries before you deep your wallet into it.
just make sure that the batt size is not more than 25MM in height coz it might not fit the rear buklhead ( if your using it in Rear Motor Config), not sure though for Mid Motor.
cheers!!!