I'd check out LiFePo4 batteries for hobby applications initially. Here's an 8.4Ah, 4-cell pack (13.2V nominal) on HobbyKing:
http://hobbyking.com/hobbyking/store/__14074__ZIPPY_Flightmax_8400mAh_4S2P_30C_LiFePo4_ Pack.html
With a 30C constant discharge rating, it should be able to crank out 252 amps continuously.