Part # CR2032...available almost anywhere...bottom side up on the install.
The battery is a CR2032... just as you find AA's, AAA's, C's, etc.
They sell them in a variety of locations, including wal-mart.
when installing the battery, the label of the battery will face up. Although in general there is a small picture nearby on the motherboard to indicate the direction.
The general theme to get from all this is to install a battery. However to revisit that you might perhaps be expecting to find a more common form of battery, such as a Double-A? it is a small, penny-sized, coin shaped battery, usually nestled in the front near the front panel headers.