Purchase your batteries from Hobby King:
http://www.hobbyking.com/hobbyking/s...s-Airsoft.html
You would be well served by purchasing more than one battery. If your only battery goes bad on you then you will be going home early. Personally, I own at least 3 batteries for every AEG I field.
A battery monitor is a must have if you intend to run LiPos (which it sounds like you are):
http://www.hobbyking.com/hobbyking/s...itor_2_6S.html
Also, make sure you pay attention to the footwear and cold and/or wet weather gear. I am always dumbfounded by players who spend mucho $ on multiple guns and super-uber-leet helmets and vests but head home when it rains because they don't have proper clothing. Your expensive gear is of no use to you if you can't keep in the game in adverse conditions!