put Airgas National Welders in google maps and look for the nearest location.
I bought a 20# from them awhile ago. Refills are like $17 with an exchange if I'm not mistaken, but I've been using my 20# for so damn long it could have changed.
I'd call them and check to see if they refill or if you'd have to trade in your tank.
Airgas near the airport filled my 10lb for $15, if that helps. They don't require an exchange, but they can't always fill it right away either. Overall I'm very satisfied with the service there.