It uses a pair of AAA cells. I work at an LBS, and we got it from J&B Importers, a distributor that most U.S. bike shops order from. They're also available on Amazon.com. Here's a thread with more info about it: http://www.bikeforums.net/showthread...s-amp-comments The one in the photo is the "Shorty" version; they also make longer ones.

I also have a BikeBrightz for my other commuting bike. They're less expensive and perhaps less of a theft target, since they zip-tie to the frame and are less obvious when turned off.