check out Hobbyking,
scroll down on the left side to batteries, select Lipo from the list.
ONce on the battery page you can select what type of battery your looking for 1S, 2S, etc. ...and narrow down your search. What is really nice is that they list size and weight of each battery (in addition to capacity and ratings) ...so you should be able to find exactly what your looking for. I've bought several from their site, happy w/ all my purchases.