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 coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I’m afraid that there’s no working coupon in Banggood.
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
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for Xiaomi SO WHITE EX3 Toothbrush Replacement Heads 2Pcs.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).