I just bought a MS setup myself and from what I gathered in researching before purchasing is that the 2 major USA online distributors are brightstonesports and lightjunction. They both sell battery packs by themselves backed by an included 6 month warranty. Also only 99ct shipping at both places.

http://brightstonesports.com/shop/in...ery-packs.html

http://www.lightjunction.com/MagicSh...da2eb59472ef2d

From what i understand, battery packs are interchangable with different magicshine models, unless you have the newer MJ-880. They note the model compatibility in the battery description.