I started off with Zippy's in the beginning as they were great for the price. I have always used 6S and purchased mine from Hobbycity/Hobbyking.com well before they were popular. Once they started to become out of stock more often than not, I started to slowly switch over to Nue Energy batteries.
http://www.neumotors.com/store/page12/page12.html
They just had a big sale and I loaded up on their 3300 and 3900 6S batts. They still have a few in stock in 4S, 5S, and 6S but your mah choices are limited. Personally, the Nue batts are better than the zippy's when it comes to performance as well as heat after a run.
Losi 8B 1.0, Tekno V3, Direct-Drive, 13/48, RX8, T8 1400kv
Losi 8B 1.5, Tekno V3 w/clutch (3-shoe), 14/48, MMM, Neu 1512 2Y
Losi 8T 2.0, Tekno V3 w/clutch (3-shoe), 14/48, MMM, T8 1550kv