Aliexpress
Click here… to search for Xiaomi PMA Graphene Neck Pillow on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (134.54 USD): BG10OFFAFF1
(CouponsFromChina.com tip: you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): vipupgrade8, ML15%Pro, BGPMAMP0, BGHC20, BGHC715, 502C6B, BGGR390, BGGRS2, BGPMAL1, BGBFHC12, bgshOOp10, BGHB20, bGhOnEy10, BgoFErTaSchInA10, BGSD52469, BgcLuBofErtA10, BGAFF10OFF, 7bcdf9, BG10OFFAFF, BGwhielabel, PMA0707
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Price history (for all/most countries) for Xiaomi PMA Graphene Neck Pillow is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).