I use Truerc 8000mah lipo packs in my e-maxx.
http://home.comcast.net/~truerc/custom/custompack.htm (8000mah packs about half way down the page)
I use a 2s2p pack (7.4v, 8000mah) ($75) in series with a 3s2p pack (11.1v, 8000mah) ($100) for 5s2p (18.5v 8000mah) total.
They are not the highest quality cells, but I like them so far and most other people that have them seem to as well, especially for the price. I think they're about the same weight as 12 nimh cells.
There is also the maxamps packs that a lot of people have.
http://www.maxamps.com/products.php?cat=45