Aliexpress
Click here… to search for Samsung INR21700-50E 3.7V 5000mAh 21700 Battery on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (19.32 USD): BGtorch9
Coupon (19.32 USD): BG8Alerabat
(CouponsFromChina.com tip: coupon code has to be applied in shop’s 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): BGSSLG, MSAMSUNGLG, bGhOnEy10, bgshOOp10, BgoFErTaSchInA10, BgcLuBofErtA10, BGFLS15, 4BLF, BGAFF10OFF, BGCuponation10, BGHONEY2110, BG10OFFAFF, BGwhielabel, BG10OFFAFF1, BGFLBA, BANGGOODSD, BGTorch4, 00c906, BGtorch2, BGtorch3, BGAlerabatI
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for Samsung INR21700-50E 3.7V 5000mAh 21700 Battery is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).