I ordered a battery online from "Cameron Sino," some Chinese manufacturer. I'm glad these batteries are still around, because otherwise I would have had to find a small battery with the same voltage and would have to do some soldering.

Here is the original battery. The label on the front is actually sticky, and it just sticks to the inside of the device enclosure.

Here's what it looks like in the case. The back side of the battery is visible, with the part number showing. I used the part number to verify that the battery I purchased online was the correct part.

And here's what it looks like when removed. I first loosened the glue beneath the battery with a screwdriver making sure not to bend the battery. Once it was free, I pulled the plug away from the PCB and put the battery aside.

The plastic shielding can be peeled away from the PCB to expose the circuitry. Not pictured is the scrolling wheel which would fit in the cutout near the upper right corner of the PCB. It can be unplugged easily from the PCB without damaging the ribbon cable.