It is a standard "unprotected" 18350 lithium battery with a flat positive terminal. You could, for example, use this Efest battery Purple IMR18350 (flat positive terminal) (1 pc., 18350, 700 mAh, rechargeable battery) but unfortunately it has even less capacity than the original.
Otherwise, just google 18350 unprotected flat and take one with as high a mAh as possible. To be on the safe side, you might want to check the brand of the battery by googling it. Some names have attracted negative attention in the past because of very bad or even dangerous batteries.