Hello, I had a nightmare trying to get my Co2 refilled and after a long time looking I ended up going to a local company who refill Fire Extinguishers, they will refill my 2kg tank for £12.
Its funny because most of the ads for aquarium Co2 set ups say you can get the tank refilled at "most pet stores" lol good luck with that because not one pet shop anywhere near me even know anything about using Co2 in an aquarium.