I am no expert in the field, yet this part should not be too hard. Although, I encourage some advice on the topic.

What we have here is a Lithium polymer battery (Li-po) of certain size and specs. The "352025" stands for 3.5mm thick, 20mm x 25mm in size. It should be noted, that the length includes the thin part, where the contacts are located. As for the specs, we look for a battery of 3.7V voltage, with two wires (black-ground, red-power), and no charging circuit (debatable).

If you are ok with current specs and don't want the risk, look for the same exact size and specs. I am guessing that the voltage is what matters, so no worry if you find a slightly different-sized battery.