The FBI has released several photos showing what they believe are parts of the "pressure cooker" bombs that were detonated at the Boston Marathon. Several of those photos, including the top one, show a battery that may have been part of a detonator. I could be wrong, but that looks to me to be part of a battery pack from an R/C car. See the second picture.
These battery packs are widely available, but they're not something you find in regular retail stores. The example I found comes from eBay.
