18650 Batteries are the most common batteries used in vapes. They are also cheap and can be recharged hundreds of times before they need to be replaced. Make sure you understand how to handle these batteries, don't just throw them in your pocket or purse as they can short-out and cause harm if not stored correctly.
Tip: Use a seperate battery charger to keep your batteries topped off and ready for use. Relying on your device to charge them may reduce their longevity.