I just wanted to throw this up here, because I see the question asked and answered over time.
Two places that I know of to get CO2:
This place sells CO2, and occasionally tanks. It's call Brewmaster, and they refill a 5lb tank for about $20 while you wait:
This places sells CO2, but you can't pick your tank. As I understand it, you can keep your tank if you ask nicely:
I don't plan to ask to keep my tank there because they still stick a nasty label on it, and it's not that cheap. Also, they are only open during normal business hours, so that may not be convenient for me in the long term.
I forget how much Brewmasters charged me for an Al 5lb tank, but I think it was about $85.
The welding supply company charged a little less than $100 for the first one, and exchanges are also about $20.
I hope this helps someone someday.