you can buy CO2 at welding supply shops. much cheaper than soda stream, of course you need to pay the $100 dollar deposit for the cylinder. most shops also have "beverage grade" CO2, which is probably less pure if anything than the regular labeled CO2, both are fine, and will likely give you a tank with a siphon if you ask, which you'll want. If your cylinder doesn't have a siphon, you'll need to hold it above your tank, upside down, which is a minor inconvenience with a 10lb or larger tank..

from there, either get a hose and adapter which connects directly to your soda stream from the cylinder, or some hoses and adapters for filling your existing CO2 tanks. paintball gun tanks can also be used with an adapter, though they'll be wider than the sodastream ones, and might not have the clearance for your machine, or wont let the plastic cover go back on.

Most Soda-stream tanks have an anti-backflow valve, which requires going slowly and carefully to fill up, or replacing the valve and then you can fill it up normally. freeze your tanks before filling, and you'll be able to fill them up more. weigh them while you do it, the cylinders will have a rated weight and tare engraved on the neck.

also, the route with the fewest tools, is to unscrew the valve, and funnel in the appropriate weight of crushed dry ice, and then screw the valve back on. sounds more expensive in the long run, and potentially dangerous.