CR2032 Lithium Cell
*Edit* FYI you can buy these almost anywhere. Wal Mart, Lowes, Home Depot, most electronics stores, etc. It's a common battery used in tons of things ranging from computers to car keyless entry clickers to garage door remotes to toys, and countless other devices. And another FYI, it's located on the motherboard behind the PCI-e slot, next to the south bridge.