RC Hobby shops are always a good bet, Airsoft and RC use the exact same batteries, chargers, and connectors (usually) so the staff there will know their stuff and be able to help you pick the best you can get.
There's also cheapbatterypacks.com is if you're willing to order from online.
PS: Sounds like you're waiting to get a gun from the classifieds first. In that case DO NOT buy a battery until you get your gun and figure out how you're going to mount it. If you have an idea of what model you want then figure out where to mount the battery then buy it.
ie. an M16 can store the battery in the fullstock in the butt area and holds a "large" battery, while an AK you can store it under the dust cover but it will only be a "stick" battery, then if you want an M4 with a MOD stock or something you can fit a "nunchuck" battery in there, as well you can also have your gun front wired to the handguard and it will fit a stick battery as well.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE
Last edited by L473ncy; April 3rd, 2010 at 15:21..