Oxygen cylinder dimension: φ6×35.5cm (2.4×14″), Breathing Oxygen Cylinder, Max. dwell time underwater: 10mins, Max. pressure: 3000PSI / 200bar / 20MPa, Material: Aviation aluminum, Air capacity: 3.0 Cu.Ft / 85 Litters, Oxygen cylinder weight: 1040g, Capacity: 0.5L.
Basically copy coupon, view price history or create e-mail price alert. You probably want to know what is the best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. You can get coupon down below.
Link: Banggood product page
1. ($210.99): 7bcdf9
apply it in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): BGSSDK, BGCE18, BGblack5, BGDH16, vipupgrade8, bGhOnEy10, BGbeauty001, BGWT063, BGHB20, BgoFErTaSchInA10, BgcLuBofErtA10, BGAFF10OFF, BGCuponation10, BGHONEY2110, BG10OFFAFF, BGwhielabel
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for SMACO Scuba Diving Reserve Oxygen Cylinder. The lowest price was: $119.9.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).