I hear dbc marine in Richmond is the cheapest, but I haven't used it. Actually, Royal City Fire Safety is really fast, but it's in New West. Way cheaper than KMS too. KMS is easy and fast, but seems to be the priciest. Bevgas in Coquitlam will do it too, but they either do exchange or only fill when they have enough cylinders to run the CO2 generator.
Any welding supply shop. Look in the phone book there are more around then one would think. But they rarely advertise, as most of their business is supplying truck and fabrication shops and not retail... But they will be more then happy to take your money too!