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 (12.45 USD): BGAFF10OFF
Coupon (12.45 USD): bGhOnEy10
Coupon (12.45 USD): BGdragonBoat
Coupon (12.45 USD): bgshOOp10
Coupon (12.72 USD): Parts8
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply 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): 502C6B, Charger12, AFfbG10, BG10BC, BGMPB2020, BGBC1, BGRCBC7
(probably won’t work, but you can try them out)
Price history and price alert
Price history (for all/most countries) for Feiyu Tech 26650 3.6V 5000mAh Battery is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).