I buy most of my new cells from Queen Battery.
Ms. Xin Wang is the Queen of Queen Battery and I have found her to be 100% honest and fair over the past 6 years and many thousands of dollars of transactions. I have recommended her to numerous people and have had no issues. Email her here.
Queen Battery also has an Ali-express store.
I am very impressed with the bang-for-the buck of Queen’s new 26800 cells. These cells have an impressive 25 watts of power each and are very well priced for their capacity. They are good looking to boot. My 26650 Blocs hold them.
For repurposed batteries I like Battery Hookup.
They specialize in cells that they have pulled from surplus or overstocked or discontinued batteries. Their cells tend to be lower capacity- a few years out of date- but very well priced. They also carry 26650 and LiFePo4 cells. They are USA based.
For small orders of high capacity cells I like BatteryBro.
They ship from the USA too and have a wide range of high quality cells at fair prices. Their site also has lots of information.