Just go to a welding gas store or resto supply store and get 'beer gas'. CO2 is CO2.
You can buy an adapter to connect the big cylinder directly to the sodastream. I have been using mine this way for 3 years. I pay about 40 bucks for a tank of CO2 that lasts ~5 months.
I used to be a HEAVY pepsi drinker but as I got older thew sugar didn't agree with me so I replaced it with sparkling water. I was going through a sodastream cylinder every 3 to 4 days. Whereas this bad boy costs me 40 bucks, and I have a 40 dollar deposit that I'll probably never get back because I will always have a cylinder.
The savings are huge.