Sodamistic - They have larger canisters with adapters for most Soda Stream models. They also have their own carbonation system. Ships out of Canada.
Soda Center - They have refills + diy home solutions for soda. Ships out of Canada.
Home Brew Supply - Online company that offers different carbonating kits (with and without canister). Ships out of America.
Ebay Option - Most expensive system, seems no different then Home Brew Supply.