Aliexpress
Click here… to search for SMACO Scuba Diving Reserve Oxygen Cylinder 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 coupon right now! Check this out.
Link: Banggood product page
Coupon (210.99 USD): 7bcdf9
(CouponsFromChina.com tip: just apply this code in your 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): BGSSDK, BGCE18, BGblack5, BGDH16, vipupgrade8, bGhOnEy10, BGbeauty001, BGWT063, BGHB20, BgoFErTaSchInA10, BgcLuBofErtA10, BGAFF10OFF, BGCuponation10, BGHONEY2110, BG10OFFAFF, BGwhielabel
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Here is price history (for all/most countries) for SMACO Scuba Diving Reserve Oxygen Cylinder.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).