1x SONOFF® DS18B20 Waterproof Temperature Probe (Cable length: about 100cm).
Basically copy coupon, view price history or create e-mail price alert. Best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. Only one coupon available!
Link: Banggood product page
1. ($12.88): ADMITAD2212
you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/i1u
Old coupons (from the oldest to the newest): elec, e2bcea, SONOFF15, BGVDay10%off, BGMPB2020, BGTT10, bGhOnEy10, BGSMOFF, BGSONOFF15, BGELIG0920, BgcLuBofErtA10, BGSONOFF16, BGSTHO15, BGAFF10OFF, BGHONEY2110, BGSHOM15, 13f142, 6GSGIT, BG15Sonoff
(probably won’t work, but you can try them out)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – SONOFF DS18B20 Waterproof Temperature Sensor [EU] ($-3)
Price history and price alert
Check out price history chart (for all/most countries) for SONOFF DS18B20 Waterproof Temperature Sensor. The lowest price was: $4.16.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).