Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong.
Link: Banggood product page
Coupon (29.99 USD): BG2f1afa
Coupon (30.59 USD): BG4ee866
Coupon (30.99 USD): BGc97422
Coupon (33.83 USD): BG00c906
(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): BG4937, BGHU980, BGF1937, BGF1493, BGAFF10OFF, BGF2299, BgcLuBofErtA10, BGCF1499, BGOCLEAN2, BGOCF1, BGF14937, BG19561, BG22371, BGWT1499, BG10OFFAFF, BGMAY048, BGwhielabel, BGjune10, BGjune21, BGjune28, BGjune50, BGjuly63, 13f142, BGjuly0770, BG10OFFAFF1, BG73202c, BGa20c4f, BGcd5bb8, BG8b78ee, BGHealth06, 00c906, BGHealth10D, 2022ACTIESNL, BGHB218, BGXPAP263, tRaDetRaCKeR, BGHECARE1, BG39a0fe, BGab19c1
(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 Xiaomi Oclean F1 Sonic Toothbrush.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).