Aliexpress
Click here… to search for RadioMaster TX16S Mark II AG01 4-IN-1 ELRS RC Controller on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (332.99 USD): BGRCT10
Coupon (340.39 USD): hoNEYbGdeAlS
(CouponsFromChina.com tip: just apply this code 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): 8BGwbg2204, tRaDetRaCKeR, 8wEbgaInS5, pROMo8cuPoN, BG8RCToy
(there’s a little possibility that old coupon will work)
Price history and price alert
Check out price history chart (for all/most countries) for RadioMaster TX16S Mark II AG01 4-IN-1 ELRS RC Controller.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).