Deadshrimp18350's are available but are pricey. Expect to pay between $7 to $11 for a Nitecore branded cell from US based online retailers like BatteryJunction or Amazon. Good news is that it will also use 16340's, RCR123's, CR123's as well. You do give up some runtime with those cells since they are lower capacity.
DeadshrimpYou can also buy an 18650 extended battery tube from elsewhere that is made specifically for this light and use any high drain 18650 you'd like. That's my plan.
iamidahoThis place is more competitive than Amazon or Battery Junction for batteries, and they are located in the U.S........ https://www.imrbatteries.com/