@Pete, there are two products that I can vouch for their quality since they have been reviewed by trustworthy people and are not expensive.
There is EasyAcc 20Ah 4 USB ports power bank. It has been tested to actually be 22 400mAh, so good thing there:
https://www.amazon.com/EasyAcc-20000mAh-Portable-Recharge-Capacity/dp/B01422TC14?keywords=20000mah+powerbank&qid=1527979650&sr=8-11&ref=mp_s_a_1_11
There is also Romoss. It is the OEM of many powerbanks such as Ravpower.
Their flagship monster has an 8A output limit over 4 ports:
https://www.amazon.com/ROMOSS-20000mAh-Portable-Capacity-Micro-USB/dp/B078JSJHPK?keywords=20000mah+powerbank&qid=1527979748&sr=8-29&ref=mp_s_a_1_29
Then there is iMuto. They specify in 26650 power banks:
https://www.amazon.com/20000mAh-Portable-Charger-External-Black/dp/B01191S0VY?keywords=20000mah+powerbank&qid=1527979849&sr=8-49&ref=mp_s_a_1_49
I can trust that last one too since I have tested it to be 91% efficient at 5V 1A, and 89% at 5V2A. Which is quite good.
BTW, guys, the reason I only buy power banks with cylindrical cells is because if the power bank dies before warranty ends, I can just ask for another power bank from the brand. It happened once with my RavPower QC 3.0 20000mAh one, in which one of the USB-A ports broke, and caused a soft short, emptying the bank in about 2 days.
After I received the new powerbank, I opened up the pack, and found 6x3400mAh cells, and I now use them in many of my flashlights.
TLDR: Only buy 18650/26650 power banks so you cab salvage the cells later if something goes wrong.
Finally, Pete, don’t sorry about some of their prices. With the 10000s of cells they are ordering, they must be getting them at 1.35-2$ a piece, then picking the cheapest case and electronics for the ones <20$, and much more efficient and powerful >30$.