I found a well reviewed US-based seller selling in bulk and went with a brand that generally isn't counterfeited that often (there exist counterfeit Samsung wrappers, but I don't think they're terribly common). A lot of the popular ecig battery brands are literally just rewrapped used cells, so I avoided the popular chemistries for them.
At some point, there's only so much you can do. You can also check the weight of the cells listed against the spec sheet, and against what is known to be possible. Any "5000-6000mah" 18650 is utterly rubbish, since nobody can actually do that.
I'll work on adding links for parts to the posts. They were mostly from transient sources, so I'll see if I can link to searches or something.
