Mostly independent welding supplies sell and fill customer owned cylinders in all sizes. The large welding supplies like Praxair, Airgas, etc, only sell smaller size cylinders up to about 80 cu. ft. They make their money on leasing cylinders. It is not uncommon for customers to acquire cylinders that should have been returned because they were originally leased. This happens when cylinders are purchased at an auction for example. Getting what you think is a bargain is often money thrown out the window because you can't exchange the cylinder without it being tied to a lease.
The high pressure cylinders are all the same (CO2 being the exception) cylinder except the valve is different depending on the type of gas. In order to switch a cylinder from one gas to another, the cylinder is purged before filling. Cylinders should be purged all the time but sometimes this step is missed. The most common "dirty" gas and most noticeable is argon. You notice it when TIG welding, especially on aluminum. There will black on the weld and even pinholes in the weld from non pure argon.