most any sporting goods store, also any paintball shops or paintball fields. for the 20 ounce bottles cost about $5-$8 to refill. there is no way to hook them up to a timer that I have found. There is no gauge to determine how much co2 is being discharged into your room. on that same sight under...
If you are willing to spend a little bit of money a good CO2 solution is a co2 tank from a paintball gun and a "remote hose" so you can safely discarge the canister. If you grow in a closet you could even drill a hole and use the remote fittings to route the co2 right into the closet.
Here are...