Batteries for the E-Power should last you quite a while before needing replacement. I use the 14650s supplied by Smoktech for the E-Power and I bought some more: Trustfire protected 14650 blues so I can charge a batch of batteries on the weekend. Trustfire Blues work great. They cost me about $4 apiece from Deal Extreme.
I recommend Hoosier as a supplier for the E-Power 14650 because he stocks it with the blue LED switch electronics. There have been many problems reported with E-Power 14650 equipped with the white LED switch electronics.
I love the E-Power 14650 and have two plus spare parts if ever needed. It's a very good performer.