Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (16.42 USD): cbrIDgTec
Coupon (16.42 USD): BGSM30%
Coupon (16.83 USD): BG1128
Coupon (18.48 USD): BGTORCH
(CouponsFromChina.com tip: you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): FIN5500, linepayflt, EYBG, BGBSPX, AC19, BG13TH, BGF1111, BGBFCR41
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Price history (for all/most countries) for 2x Sofirn 3.7V 5500mAh Protected 26650 Battery With Case is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).