We just started avidly using Sodastream and realized that you go through a cartridge for 60L pretty rapidly when you're carbonating multiple bottles a day. I looked up replacement cartridges and saw that it's $15 to exchange an empty cartridge for one filled with CO2 which seemed exorbitant.
More OCD research revealed that lots of people buy an adapter (~$20 on Ebay) to the Sodastream cartridges that connect to paintball CO2 cartridges which also have the added benefit of lasting much longer, but it seems difficult to find a place to refill these cartridges. Sports Authority has horrible reviews about their CO2 refill service on Yelp and the paintball cartridges seem too small to take to an industrial welding shop. Any thoughts on how best to get them refilled around Berkeley?