We use S&S Carbonic, 124 16th St. South 205-323-1626. They filled a 5 or 10# for $5, 20# for $10. The guys that fill the tank like cash. I have heard the they have recently changed hands and prices have increased but have yet to go in myself to see what they are now. I have heard also that they now charge a fee to have a tank filled while you wait.
Tomoko - that is a good price for a filled $20 tank, less than wholesale for the tank alone. Does your employer have a contract with them?