I made a comment that I could paint the Mystic logo on a battery pack when Pokemon Go came out.
I started with one of these battery packs off of amazon, they can be found cheaper on other sites china direct but you will have a multi weak lead time.
Its great if you use 18650's for other electronics, most notably being flashlights or vapes. I use them to power all of my electronic projects or to build custom battery packs. I am using 4 Panasonic NCR18650B batteries, they have a massive 3400 mah rating but any 18650 that can put out at least 3 amps of current draw would be fine.
Step by step gallery.