Aliexpress
Click here… to search for Matek F405-STD BetaFlight STM32F405 Flight Controller FCHUB-6S on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. Well, there is one coupon available right now.
Link: Banggood product page
Coupon (44.62 USD): Parts8
(CouponsFromChina.com tip: this code has to be applied 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): BGMatekNEW, BGSFP4, AFfbG10, BGVDay10%off, BGMPB2020, BGAFF10OFF, bGhOnEy10, BGdragonBoat, BGSS10%SW, bgshOOp10, BGRC902, BgoFErTaSchInA10, BgcLuBofErtA10, BGCuponation10, BGDAVIS, BGHONEY2110
(these coupons are old, so probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for Matek F405-STD BetaFlight STM32F405 Flight Controller FCHUB-6S.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).