You can create price alert here, copy coupons and view price history. What is best price?
Waterproof Rating IPX7, -30s Intelligent Reminder, -DuPot 0.15mm ultra fine bristles, -31000 Acoustic Vibration, Working Time 30 days, -Wireless Inductive Charging, Rated Voltage DC 5V, -Food Grade Brush Head, Weight 250g, Charging Time About 10 hours, Power 2W, -4 Brushing Modes
Aliexpress
I could not find it on Aliexpress. Click here to search by yourself.
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
1. ($26.03): 13f142
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 and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): TTxBG10, aff7off, 2019SPEDM, bgssaff9, AFfbG10, BGBGHFRI15, 0e7f33, BGHOMEG12, FBCOUPON1, BGHome15%, BGHOMEA15%, BGAFF10OFF, BGCuponation10
(these coupons are old, so probably won’t work)
Price history and price alert
Price history (for all/most countries) for Mr.Handx Sonic Toothbrush is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).