Aliexpress
Click here… to search for 2x Screen Protector Film for Mi Band 3 on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Price history and price alert
Price history (for all/most countries) for 2x Screen Protector Film for Mi Band 3 is here.
Price records source(s): Gearbest
Click on image to enlarge. 
You can set price alert (for Banggood).