Aliexpress
Click here… to search for OnePlus 6T 8/128GB Midnight Black on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong.
Link: Banggood product page
Coupon (369.99 USD): BGUSOP6T
Coupon (450.99 USD): 7bcdf9
(CouponsFromChina.com tip: put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): 11BGR6P4, 11BG6T8, BG116T8, 6T11BG8, 12bgop5, 12MCOP6T8, 1BGOP6T2, Alibuy9137, 1BGOP6TA, 2BGOP6TB, 62ca20, 3BGOP6T, JPBG128W, koninimbesinop6t, f262fb, JPBG128h, plfonall, 19BGOP6T, 4OP6T, 4bgop6t, 5BGOP6T, bg845, BG5OP6T, BG6Tplus01, BGDJOP6T, BGOP6T527, BGONE11, BGaDmITAd
(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
Here is price history (for all/most countries) for OnePlus 6T 8/128GB Midnight Black.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).