Some complexities but a good place to start:
Types of lithium batteries

Also this guy from Electronic Cigarette Forums (ECF):

Goes by the name Mooch and here’s a link to his page:
List of battery tests

That last one is rather extensive but can be used for reference. Mind you he’s testing for high drain capabilities as their “mods” are for vaping.
Personally, stick with the known reputable brands as Samsung, Sony, and LG. However, many counterfeits of these - often on china sellers.

Edit: If you don’t care or want to track down a good seller, try a vape shop. They always have top quality cells but at a higher cost.
Depending on usage (cycle times) you’ll need a good charger:
Lygte Battery Chargers Review