Maximum power of normal version: 5V-2.1A, Applicable battery: 18650 battery (length 6.5cm / straight down 1.8cm), Model: C16.
Just check coupons out (with price alert and history). You probably want to know what is the best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. Only one coupon available!
Link: Banggood product page
1. ($12.96): BG00c906
apply code in your basket (cart) after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): ADMITAD2212, 2022ACTIESNL, Terence, Tabby20, 2203ActiesNL, BG1efeb0, MVC8SPRING, BGJUNEGSG9, Cuponation9
(probably won’t work, but you can try them out)
Price history and price alert
Price history (for all/most countries) for C16 Removable 16-section Power Bank Shell 18650 5V 2.1A is here. The lowest price was: $7.88.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).