Aliexpress
Click here… to search for OnePlus 6T 8/128GB Mirror Black on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in Europe.
Link: Banggood product page
Coupon (369.99 USD): BGOP6T8128
Coupon (469.99 USD): affiliate6
(CouponsFromChina.com tip: this code has to be applied 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): 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, BG20%OFF, BGOP6T8, BGOP6T8G, aff7off, BGOP6T628, BGOP6TIL, bgssaff9, BGletni16T, BGOP6T79, BGOP20, BGOP6T719
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Here is price history (for all/most countries) for OnePlus 6T 8/128GB Mirror Black.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).