18650 LiIon batteries are used to power flashlights and the like. I use them for inexpensive bike lights. There is a baffling array of 18650 cells for sale at a truckload of places. The difficulty is knowing which are good and which are not.
I have used the Trustfire 2500mAh blue label batteries for several years. The capacity is more like 2000mAh (instead of the claimed 2500mAh capacity), but they have proved to be reliable.
The best value for money / capacity readily available seem to be the Trustfire protected "true" 2400mAh batteries with the black and red labels .
Currently, the highest genuine capacity 18650 cells come from Panasonic with their NCR18650B 3400mAh batteries.
Panasonic also have a series of 18650 cells with 3100mAh capacity. Not quite as much as the 3400mAh cells, but with a higher discharge rate potential.