@klardotsh We bought a 5kg co2 tank and regulator in 2019 while in Japan for fizzy water (helps to keep water interesting in hot summers w/o cold water).
Gave the big tank away and switched for a paintball tank (a friend gave us one of his), easier to re-fill/carry :D! It's the perfect size.
You can get adapters for sodastream systems too I think, if you dont want to use their proprietary bottles. I saw some when trying to find the adapter for my system.