These batteries are very common for flashlights and vaporizers. I use them for both. I technically have 5 choices for charging these batteries from DC, but I'm going to skip the last one because it's a terrible option and I don't feel safe using it, so therefore I never do.
Chargers for 18650 cells:
I have a no-name single cell 18650 (no other size lithium cells fit) USB charger. I don't use it often, but keep it in my backpack at all times as an emergency charger. Also has USB output to charge USB devices from a single 18650 cell.
This is another battery that I could theoretically charge with the X1, but have not personally done because I do not have a battery sled to adapt to the X1. Since I have plenty of other charging options, I haven't wanted to spend the money for a sled.