I remember looking into this about 2 years ago but was still very sick at the time.
Did lots of research on it too. I agree with the bafang and I also have a resource I can give you IF you want to build your own battery.
From memory I was looking at fasttech for batteries as they have bulk purchase pricing.
But looking through the thread here there’s plenty of good advise. Good luck with it you ave just re-ingnited my want of one 
The book is called DIY Lithium Batteries How To Build Your Own Battery Packs.
If anyone would like a copy pm me and I’ll send it to you