Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. We have found one coupon for this product!
Link: Banggood product page
Coupon (628.00 USD): 13f142
(CouponsFromChina.com tip: you should apply the coupon 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): BGCuponation10
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for OnePlus 8T CN Version 8/128GB 5G is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).