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Instead, buy a cell enclosure. These range from a single cell enclosure to 6 cells. More is better although they don't include a BMS so some means of occasionally balancing the cells is a good idea.
Basically this is like the sealed powerbanks you can buy except it's user serviceable. You can open it up and get at the cells, no soldering required. This means if a cell dies you just pop the case open, find the dead one and swap in a fresh cell.
It's also much cheaper to do it this way for the same reason it's cheaper to build your PC rather than buying prebuilt Alienware shit.
Now, what cells to buy? Generally you can trust major manufacturers like Panasonic and LG. Steer clear of Ultrafire! That's an altogether fraudulent brand. They make most of their sales off people who confuse them with Trustfire. Ultrafire cells are usually 1/10th of their rated capacity and the seller will fight your efforts to get a refund. If you open one of 'em up there's just a smaller cell and a filler material inside like sand.
The best cell you can buy at the moment is the Panasonic NCR 18650B. They sell them in multiples of 2, so buy an enclosure that holds 2, 4 or 6 according to your needs. Pic related, that slipped my mind and I bought one that holds 5, now I have a loose cell laying around. Oh well, that'll be the replacement if one of the cells in use atm goes bad.
You can find an enclosure by searching Amazon for "6x 18650 battery box" where you replace 6 with however many cells you want it to hold. You can find the cells by searching "Panasonic NCR1850B". Make sure you read the reviews to ensure the cells are legit before pulling the trigger.
Now feel free to ask me about all the other options I didn't touch on, and I'll tell you why I didn't.
