Aliexpress
Click here… to search for Digoo DG-HOSA 433MHz Water Leakage Alarm on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. One coupon is available. Cool!
Link: Banggood product page
Coupon (13.59 USD): BGSmart6
(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): affiliate6, bgssaff9, aff7off, BGaDmITAd, bgchinafest, BGDigoo15%, BGDG15, BGSAMH, 20%DG422, BGSMOFF, BGDGASG, BGSAMRTH, BgcLuBofErtA10, BG15SH12, BGSTHO15, BGHONEY2110, BGSHOM15, BG6c046a, 13f142, BGSHE15, BGISOG753, BGedb972, 6GSGIT, BG4fcf10, BGD11073, BGf27012, BG4a4559, ADMITAD2212
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Here is price history (for all/most countries) for Digoo DG-HOSA 433MHz Water Leakage Alarm.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).