I like Airgas, it's 24$ for a 20lb swap. That tank last over a year. Total time in and out 5min. The guys there like beer (good to know). I always check the hydro dates, FYI.
I live a couple blocks away from a welding shop, and they open at 7am. I usually go before I start working Monday morning (typically I find I have run out of CO2 on Saturday evening), but they are close enough that it's not a big deal to take 20 minutes and go at any time they are open, since I work out of my house most of the time anyway.