Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. We have found one coupon for this product!
Link: Banggood product page
Coupon (80.99 USD): 13f142
(CouponsFromChina.com tip: 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
Old coupons (from the oldest to the newest): a45b01, vipupgrade8, TTxBG10, BGOCLEAN, BGCFC01, BGss8%hl, bgssaff9, AFfbG10, BGRN732, BGOCA99, 502C6B, BGBGOOD4, bgshOOp10, BGHB20, bGhOnEy10, BgoFErTaSchInA10, BGSD58965, BgcLuBofErtA10, BGHBHJ107, BGAFF10OFF, BGCuponation10
(probably won’t work, but you can try them out)
Price history and price alert
Price history (for all/most countries) for Xiaomi Oclean Air Sonic Toothbrush White is here.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).