If you want a Joye eGo 510 setup, or Riva eGo, liberty flights (and make sure to use their 20% discount) is a popular place.
If you want mod batteries, well, you should buy good ones (AW if possible).
If you want slim batteries (808's), you get what you pay for, without a doubt.
Lots of info in the forums, but you have to be a bit more specific in what you're looking for.