Same place it gets filled, if it's a commercail gas place. They will charge you for the service though. If you get it filled someplace odd, look for an AirGas outlet in the yellow pages.
Fire esinguisher places do hydros and you can just swap the tank out at some suppy places (if it's still in hydro) or pay 20bucks. I swap all my tanks out at supply places. HVAC supply shops have co2 (10lb and 20lb) tanks to swap.