I thought that welding supply places filled Co2, but I called a few in Justice and Bridgeview and they don't. I know that the paintball places fill tanks for the paintball guns, maybe you can throw them a few dollars to fill your tank. For a 20 oz tank for a paintball gun, they charged about 5 or 6 bucks. I don't know how much they would charge to fill a bigger tank. Or they may tell you where they get their tanks filled. Hope that helps.
my local gas supplier had no problem filling mine, but they didn't do it on site....took a day-two, just be REALLY sure they can guarantee that you'll get YOUR tank back to you...