Almost any 12v battery would work. I would say at a minimum 5000 mah. I run one of these.
http://www.onlybatterypacks.com/showitem.asp?ItemID=11075.37 Just make sure it is fully charged before each race. Also if you run a mychron 4 tied in to the same battery it shuts off on low voltage (below 10v I think) so if the gage shuts off come in because the water pump is not fare behind.