Aliexpress
Click here… to search for EMAX Nano 50mm FPV Antenna 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 (5.12 USD): Parts8
Coupon (5.12 USD): BGTT10
Coupon (5.12 USD): BGFPVS10
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply 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): BGBCBG10, 13f142, 502C6B, BGOTC21, BGBFAFF2019, BGCITYADS2019, 2020CNY, BGVDay10%off, bGhOnEy10, BGSS10%SW
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for EMAX Nano 50mm FPV Antenna is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).