I've been using CO2 for about a year now and have found most places that fill near me (Frederick) will usually assume I'm looking for an exchange but have the ability to fill with out an exchange a few days a week. I usually go to AirGas up here (believe they have locations near you as well) and they have a truck that comes on Tuesdays and Thursdays to fill on site. I will usually drop it off the night before and its able for pick up the next day. Runs around $15 or so to fill (10lb tank).
Ryan