Roberts Oxygen, they generally exchange tanks, check the phone book for your local outlet. I exchange 20#'s for $23 or so.
Maryland Fire Extinguishers, (I think) in Rockville, will fill while you wait. Little more expensive than Roberts is, now, used to be cheaper.
I just called them and they will sell me a reconditioned 10# aluminum for $90 filled. $20 for refills while you wait. Is this a good deal? Roberts wanted $157 for a steel tank and $22 for swapped refills. If I could find a used tank I would use Roberts as they are more convenient but it looks like I'll need to buy one so refills while I wait is the way to go. Anyone have any suggestions?