Aliexpress
There’re some similar items on Aliexpress in my database. There may be some other items though.
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. Hmmm… There is no active coupon in Banggood at this moment.
Link: Banggood product page
Price: -3 USD
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): ML15%Pro, BG15%SP, 9431a8, BGEX3R0, BGEX3R83, vipupgrade8, BGHU356, BGWHITEREPLACE, BGSOOCASSEP, BGNOV7, BG11VIPD, BGBFHC4, BGSCTF1, BGSOOCAS18, BGTMCB7, BGHU841, BGXIAOMI811, BG11025, BGAFF10OFF, BG11716, BGHUD412, BgcLuBofErtA10, BGCC0704, BGHBHJ104, BGCuponation10, BG10OFFAFF, BGAHM34933, 13f142, BG10OFFAFF1, BGdb168d, BANGGOODSD, BGHealth06, BG4ee866
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for Xiaomi SO WHITE EX3 Toothbrush Replacement Heads 2Pcs is here.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).