Aliexpress
Click here… to search for OnePlus 8T 8/128GB Global on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (459.00 USD): BG6305b4
(CouponsFromChina.com tip: apply code in your basket (cart) after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): BGOP8T128, Email5, BG8T1281102, BG8TOP128, BGDO8T128, BGWP8T128, BGBF8T128, BGBFNA8T128, BGFSIT8T128, BGBFBR8T128, BGBF8TOP128, BGBFFS8T128IL, BGCuponation10, BG3SS8T8, BGS3SS8T8, BG3SSYB8T8, 13f142, BGOP8T8D, BG1b7429, BGOP8T8G, BGa36935, BGf5A1C6, BGOP8T12825, BGaebb27, BG87d76e, BG3b63c6
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for OnePlus 8T 8/128GB Global is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).