I got an unbranded one from a market initially - actually worked fine, but it contained a battery of much smaller capacity than advertised.
I've bought one from Ebay that lasted for 2 charges!
The others I have (all also unbranded) were from a local shop and FastTech and all perform very well.
There are plenty of vendors selling good eGo style batteries, but most of them aren't selling off ebay or out of a suitcase in shopping centres!
When you're first shopping online, you can't avoid TW, as search engines will point you there - fortunately, I figured they were overpriced before I committed to my first collection of DIY ingredients (I found this place in time and bought them from the marketplace).
From what I can gather, most vendors affiliated with POTV are small companies borne out of a passion for vaping, and will offer good customer service and, most importantly, sort any problems out if anything goes wrong.
I strongly recommend an eGo & Evod set if you're looking for a new kit - a few here sell single or double sets and Evod's a much better starter tank than a CE4 in my opinion (if I could go back, I'd do most things the same, but I'd skip the struggle of getting a decent vape from the varied quality of CE4s and go straight for the Evod)