I just bought a couple from these guys, they even threw in a spare flashlight but I only bought one battery so I don't have a battery for my flashlight and now I need to go buy another battery.
Amazon has a problem with counterfeit products across the board, not just batteries.
I haven't had to buy any for a while so I don't have a place I can currently recommend, but check out the vaping subreddits, or forums. A lot of vape mods use 18650 batteries, and consequently the forums and subs devoted to vaping are really good resources for finding retailers that sell legit high quality batteries, the ones to avoid, battery safety info, and stuff like tutorials on when and how to re-wrap them if the plastic jacket gets damaged.
From someone not in the hobby: what exactly should I imagine under the term counterfeit batteries? Are those batteries that can be used in the same devices as tge real ones but with different specifications or are they just from different lower quality brands than the one advertised?
They are liable to vent (where they overheat and off-gas dangerously) and/or explode. Stay away from counterfeits like your life depends on it, because it very well may.