get yourself a decent 8.4V intelect battery from Airsoftparts.ca
Don't forget to get a charger, The wall chargers that come with most guns are craps, but a smart charger so you dont wreck your battery
This is probably the most basic affordable smart charger out there:
http://www.airsoftparts.ca/store2/in...oducts_id=1030
Look for a battery like this one,
http://www.airsoftparts.ca/store2/in...oducts_id=1069