Aliexpress
Click here… to search for AC/DC Power Supply ESP8266 WIFI Relay Module ESP-12F 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 (11.76 USD): Terence
Coupon (11.88 USD): BG00c906
(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
There were country-specific coupon codes published on Couponsfromchina social channels and price alerts. Coupon price may vary by country, because there’re different VAT rates, shipping cost etc. There is a chance that some latest coupons are still working:
Old coupons (from the oldest to the newest): BG2589f4, 00c906, ADMITAD2212, 2022ACTIESNL
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for AC/DC Power Supply ESP8266 WIFI Relay Module ESP-12F.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).