The 18650 batteries are NOT included in the package due to global supply chain interruptions, as of writing. Finding 18650s in stock may require extra searching.
There is an extensive thread discussing 18650 batteries on the Clockwork Forums.
Sizes
The 18650 comes in various sizes, and the DevTerm battery holder is compatible with those sized Φ18±0.5mm diameter, 65-69mm length. If oversized batteries are too big, they will put extra tension on the battery springs or plastic cover, potentially denting the batteries or damaging your hardware. This being said, many of the recommended batteries are a “tight fit.”
Circuit Protection
The battery module has “Short Circuit and Reverse Polarity Protection” features, but you are still advised to purchase the batteries from a reliable distributor and install them correctly. Nitecore and Panasonic are regularly cited as reliable bands that produce multiple 18650 models, some featuring protected cells to prevent accidental damage to the batteries or DevTerm.
Avoiding Counterfeits
18650 batteries are commonly used in vaping devices and may be available at local retail stores that carry such equipment. Beware of counterfeit and/or poorly made batteries when ordering online. It is best to stick with reputable brands.