Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (14.71 USD): Parts8
(CouponsFromChina.com tip: coupon code has to be applied in shop’s 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): Frskys8, 502C6B, 10Frsky, TTxBG10, BGFrsky
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for FrSky 900MHz RC Antenna for R9M / R9M Lite.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).