Aliexpress
Click here… to search for 4x Xiaomi DOCTOR B Manual Toothbrush on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (13.46 USD): AFfbG10
Coupon (13.46 USD): affbg10
Coupon (13.96 USD): 13f142
(CouponsFromChina.com tip: put item in your cart and apply coupon code there 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): 69c16f, TTxBG10, 2019SPEDM, bgssaff9, BGBFAFF2019, BGCITYADS2019, BGBGHFRI15
(these coupons are old, so probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for 4x Xiaomi DOCTOR B Manual Toothbrush.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).