For an "ego style" battery I suggest buying from reputable e-cig vendors such as AltSmoke, Discount Vapors, My Vapor Store, etc. Buy only name brand Joyetech Twist or Vision Spinner ego batteries. BTW, "ego" is merely a type of battery, not a brand name. The higher the mAh, the longer the battery life.
If you were willing to go for a small "box battery", look at the MVP. I highly recommend this as well.
1. Huge (2600 mah) battery
2. Variable voltage/wattage
3. Ability to vape while charging
4. $40 shipped at many online stores