www.rclipos.com I've used them a few times now.
Excellent service, and they don't sell crap LiPOs. The Hyperion VX range are the best value for money, whilst for ultimate performance either the Thunderpower G4 45C LiPOs or the PolyRC 43C LiPOs are the ticket.
Free (fast) shipping and good prices. If you send them an email and are planning on buying mulitple LiPOs, they'll probably give you a discount.