Aliexpress
Click here… to search for Xiaomi Oclean One Replacement Brush Head on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. One coupon is available. Cool!
Link: Banggood product page
Coupon (12.33 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): vipupgrade8, TTxBG10, 2019SPEDM, bgssaff9, AFfbG10, BGBFAFF2019, BGCITYADS2019, BGBGHFRI15, BGHOMEG12, BGMPB2020, BGAFF10OFF, bGhOnEy10, BGdragonBoat, BGSS10%SW, bgshOOp10, BGHome15%, BGHOMEA15%, BgcLuBofErtA10, BGHBHJ107, BGCuponation10
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Oclean One Replacement Brush Head.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).