I have several flashlights with 18650 batteries.  I caught some Ultrafire lights on sale and bought several.  I usually have one in my coat pocket.  The 18650 are used in some solar lights. I found some in the clearance aisle at Walmart once.  You have several choices on Amazon.  Here is an example www.amazon.com/Garberiel-Performance-Rechargeable-Batteries-Chargers/dp/B07311QK1D