I've heard about Horizon Hobby and Hobby King, but a new upcoming brand is Zeus for lipos from a website called http://www.powercellbattery.com.  The 2200 mah 20C provides awesome power in my planes but they only problem is they have a limited selection.  But their 2200 mah provides about 7-8 minute flights with power to spare.  I've had close to 20 flights on the battery so far with no power issues at all. 

But for the price on the few they have, it can't be beat.  They have about 6 different lipos right now .  http://powercellbattery.stage.thomas...ower?forward=1. Not sure if you're looking for a particular size and I know you're looking for packs for your car, but they do have a range of mahs and voltages.