Where to find CO2 cylinders (other than LFS or MO)

[Delphinus]

Wondering where people could go to get CO₂ cylinders (if not at LFS). Been trying some welding supply shops, CO₂ supply, medical O₂ supply places, but not really having a huge success rate.

Do CO₂ cylinders ever come up for sale used (there has to be a market other than aquaria, i.e. beverage dispensers)? I called a few places and got laughed at when I asked if they had (or knew of) any second-hand cylinders. Is it really all that absurd a notion?

Surely I'm just calling the wrong places! Someone tell me what category to look up in the yellow pages, PleaseAndThankYou_(tm)??