I bought an adapter and a 25 liter siphon tank.

I found a local gas company that will refill my siphon tank with food grade CO2. Because it's a siphon it'll transfer liquid directly into the SodaStream tanks.

I own five SodaStream tanks. I can refill them about four times out of the siphon tank.

The siphon tank cost $25 to refill at a semi local gas company.