I used to get my tank refilled at Red Comet. Red Comet moved, so I was on the lookout. My local liquor store has branched out into the home kegging industry.
Liquor Barn on Broadway (west side of the street, in between 285 and Belleview) right across the street from my local homebrew store (Beer at Home)
http://thedenverliquorbarn.com/
Quoted from their website:
"We also exchange empty tanks for full tanks. Bring in your empty tank, and leave with a full one!
Exchange Prices:
TYPE SIZE PRICE
CO2 2.5LB $7.00
CO2 3LB $9.00
CO2 5LB $10.00
NO2 5LB $15.00
NO2 100% 5LB $19.99
CO2 7LB $12.00
CO2 10LB $13.00
CO2 15LB $15.00
CO2 20LB $19.00
CO2 50LB $40.00
NO2 50LB $69.99
Well give you trade-in credit toward a filled tank, or for an upgrade charge, you can trade up to a larger tank, or upgrade a steel tank to aluminum. Some conditions apply.
If your tanks stamp is older than five years, you will need a hydrostatic test, and an extra $15.00 charge will apply. We dont always have every size, so if youre looking for a specific size, please call first to make sure we have one available. We always have lots of tanks, and will always work something out with you."
They have always been very nice and helpful to me. The last time I needed CO2 I traded in a 4 year old tank (in need of a hydrostatic test soon) for a tank that was certified within the previous 4 months.
Good Luck!