A lot of players on ASC (Airsoft Canada, one of the largest airsoft boards on the planet) recommend CheapBatteryPacks.com
. They'll even custom make one for you if you need it. As far as parts go, I get most of mine from Ehobby Asia
. Of course, there's always Ebay. Being that airsoft is much more politically correct in the USA than Canada, there's also a wealth of brick and mortar shops there as well.
I could probably dig up a bunch of US shops, but the players here would have more experience than I would. The only US shop I've dealt with is Airsoft Extreme. Several players here have good things to say about Airsoft GI. Evike is considered to be one of the worst businesses to deal with up here, though they seem to have a good name in the US.