Aliexpress
Click here… to search for MantisTek GM2 3500DPI Mouse on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. You can get coupon down below.
Link: Banggood product page
Coupon (17.99 USD): 13f142
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): 215c65, f3e3a0, 502C6B, TTxBG10, e18b44, BGe28b38, AFfbG10
(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 MantisTek GM2 3500DPI Mouse is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).