|Li-Ion batteries used to be very expensive and complicated to handle. There's a moderate risk of explosion when they are treated wrongly. Nowadays, they are widely used in cellular phones. Cheap, Chinese-made replacements can be found at online stores and night markets. Some of these cheapies are not bad. Often they include a charge-/discharge control circuit that prevents explosion when being charged by 5V constant voltage. The circuit also eliminates the risk of destruction from deep-discharge.
I purchased a number of identical Li-Ion batteries for a popular Nokia phone, then connected them in parallel. Works fine, holds a lot of energy for its weight. See the picture to the left.