I have just checked coupons. The lowest price I could get is 13.99 USD for Diamond Sharpening Stone 600/1200 (in Banggood). Basically copy coupon, view price history or create e-mail price alert. Let’s see what best price is ATM.
Latest coupon and price update (UTC):
Banggood (13.99 USD)
Banggood warehouse is located in China. Too bad there’s no coupon right now in Banggood. What about the price? Good price! It’s lower than average one (14.45 USD)!
Link: Banggood product page
Price: 13.99 USD
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGWDT15, BG76cfa1
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Aliexpress
Click here… to search for Double-Side Base Diamond Sharpening Stone 600/1200 Grit on Aliexpress
Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for Double-Side Base Diamond Sharpening Stone 600/1200 Grit.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).