Just want to give a quick shout out to Denver Syrup. Went and refilled my CO2 bottles there a couple weeks ago.
I had bought some bottles off craigslist that another vendor would not refill. Denver Syrup gladly took them though.
Denver Syrup is family owned and small. Very reasonable prices ($28 bucks for a 20 and 5 pounder)
If you're having troubles getting your CO2 filled by people, take your bottles there.
They also refill your CO2 while you wait (10 mins max)