Bought my replacement battery from Amazon. At the time it was $10, now it's $13. I suggest you also buy replacement skates because in order to open up the mouse you will need to unscrew some screws located underneath the pads, and they'll be ruined in the process.

Good thing about that battery is it's actually got more capacity than the original. So it lasts longer. Like, the lights on the mouse will turn red indicating low batt, but it will still keep going for like a week.