Aliexpress
Click here… to search for 1x IMALENT MRB-186P30 18650 3000mAh Battery on 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 (15.12 USD): BGHONEY2110
Coupon (15.29 USD): 4BLF
(CouponsFromChina.com tip: this code has to be applied in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): MIC86, MIMRB, linepayflt, EYBG, BGTORCH, 502C6B, BGBFCR41, bGhOnEy10, bgshOOp10, BgoFErTaSchInA10, BgcLuBofErtA10, BGFLS15, BGAFF10OFF, BGCuponation10
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Price history (for all/most countries) for 1x IMALENT MRB-186P30 18650 3000mAh Battery is here.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).