Aliexpress
Here are coupons that you can use.
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – SONOFF DS18B20 Waterproof Temperature Sensor [EU] (-3 USD)
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (12.88 USD): ADMITAD2212
(CouponsFromChina.com tip: just apply this code 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): elec, e2bcea, SONOFF15, BGVDay10%off, BGMPB2020, BGTT10, bGhOnEy10, BGSMOFF, BGSONOFF15, BGELIG0920, BgcLuBofErtA10, BGSONOFF16, BGSTHO15, BGAFF10OFF, BGHONEY2110, BGSHOM15, 13f142, 6GSGIT, BG15Sonoff
(I don’t think these’re gonna work, but you can try ’em out anyway)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – SONOFF DS18B20 Waterproof Temperature Sensor [EU] (-3 USD)
Price history and price alert
Check out price history chart (for all/most countries) for SONOFF DS18B20 Waterproof Temperature Sensor.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).