cells can be salvaged from old laptop batteries (like I did) or can be bought separately. As our aim is to build a cheap but good , I recommend to search and find an old laptop

Cossatot 1000 XL Duty USB .

Non- vs. Batteries. Why Batteries? Things To Consider When Choosing For Your . 18650 rechargeable flashlight

rechargeable flashlight 18650 | eBay

18650 Rechargeable Batteries – Walmart