See my post (#11 01-15-2014, 10:45 PM): I'm still refilling my SodaStream CO2 tanks using dry ice. You don't need any adapters or additional cylinders. It's food grade CO2 and only takes about 10 minutes (which I'm guessing is less time than you spend waiting on someone to refill a cylinder at...
I use dry ice (aka frozen CO2) from the grocery store to refill the Soda Stream. To my knowledge, dry ice from the grocery store is food grade (since its intended to be used for that), and only costs about $2/pound. The 60L Canister holds 1 pound of dry ice, while the 130L holds two pounds...