A friend of mine (actually a friend, not me, I have known forever not to buy soda streams I PROMISE I'M NOT A LIB 
) wants to actually try to boycott Israeli products (finally). They have a soda stream though and they want to continue to use it, but they don't want to buy the cartridges from soda stream because they're don't want to support genocide. Since this is a noble goal I want to help them.
So what are their options, where do they buy cartridges for their soda stream that don't support literal genocide. This is their Very Important Treat and they really want their treat and we all deserve our treats so I'm trying to help them. Thank you.
Thanks everyone this was super helpful...looks like you just buy an adapter and then I've got a bunch of local brew stores that will fill the tank!
My parents got an adapter from Amazon that allows them to attach an actual standard CO2 tank, purchased at a local gas company, to the soda machine, which they get refilled every so often. The gas company also refills used SodaStream canisters, so they also get those recharged to use as backups when the standard tank runs low.