You would need a 16340 battery. It is a 3.7 Volt, Li-Ion, rechargeable battery the same size as a CR123A. Do not buy an RCR123A. That is rechargeable also, but they are 3.0-3.25 Volt batteries.
Note: There are a lot of RCR123A's sold that are incorrectly labeled "16340". Just make sure it says...