Generally you can't buy CO2 tanks that are already filled unless you buy them from a local company like Airgas, because when it's full it's a hazardous material and lots of extra fees for shipping.
You might be able to find local places that refill at
http://www.gawda.org/ .
Otherwise, try local beverage supply companies, home brew supply places, or fire extinguisher supply places.
You can also get full canisters at Airgas, but they don't refill, so just make sure you don't have a pretty new aluminum cylinder if you take it there because they will just give you whatever rusty old steel cylinder they have laying around.