Took 2 minutes to disassemble both to find that the Hotshot 2W and the Hotshot PRO use the exact same battery pack.

Li-ion 3.7V -- 3.1Wh 830mAh
Model: J139
28mm H
39mm W
8mm D

Battery appears to have a typical JST micro 3-pin connector. I haven't been able to find a matching item yet, all of the results keep bringing back LiPos, and I'd rather not have my light explode, as there is no built-in discharge limiting circuit.