Aliexpress
Click here… to search for Xiaomi SOOCAS X3/X1 2Pcs Toothbrush Heads Black on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (31.41 USD): BGHealth06
(CouponsFromChina.com tip: just apply this code 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): 502C6B, vipupgrade8, TTxBG10, BGВСЕHead, aff7off, BGkoncowki, BGSORX3, BG15%SP, affbg10, AFfbG10, BGSOOCASSEP, BGsoocaskon, BGSX3TH, BGSOOCAS18, BGTMCB7, bgshOOp10, BGHB20, bGhOnEy10, BgoFErTaSchInA10, BGSD58965, BgcLuBofErtA10, BGHBHJ104, BGAFF10OFF, BGCuponation10, 13f142, BGHONEY2110, BG10OFFAFF, BGwhielabel, BG10OFFAFF1, BANGGOODSD, 6GSGIT
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Price history (for all/most countries) for Xiaomi SOOCAS X3/X1 2Pcs Toothbrush Heads Black is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).