Aliexpress
Click here… to search for Eachine TriangleD 5.8G 40CH FPV Transmitter on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (17.09 USD): BGEachine2021
(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): BGTriangleD, BGEachineFB, BGET58G1, BGET58G2, BGET58G3, BGET58G4, bGhOnEy10, Eachine11, bgshOOp10, BGRC902, BGTT10, BG11EACHINE, BGEachine10
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for Eachine TriangleD 5.8G 40CH FPV Transmitter.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).