The orange waterproof match holder from Walmart holds a single 18650, and many of the cheap little belt holsters for small pocket knives and such will hold one, some will hold two.
You can get the adapter sleeve that is used for some 3AAA lights that also use 18650s, to put inside the soft holsters as hard protection for the battery.
I even have a slender but long belt holster that I can put one battery in the adapter sleeve first, and then stack the match holder on top, which gives me two batteries.
Fasttech sells the adapter sleeves.