Check around for bottled gas suppliers. You might also look around for home brew stores.
If you've got a home brew supply store around, I know they'll have CO2. I have a 20 pound tank that I use to carbonate beer with. I don't use it on my tank, but I know where to get them if I needed something for the tank. The brew house typically has 5 pound, 10 pound, and sometimes 20 pound tanks. They should also have lines and regulators. I don't know if you need the same type of set-up for a fish tank. But, all the supplies can be found there.
CO2 is much better when used for beer purposes. :thm:
Joe