www.aquariumplants.com sells
CO2 cylinders...prolly worth comparing their price to somewhere locally, just in case.
I would try your welding shop first. They may have a leasing program, where you put a deposit on the cylinder, and then just swap it and pay for a fill when it's empty.
Owning your own cylinder has some disadvantages...places tend to only fill them on weekdays, and you have to wait for the fill. Every few years it needs a saftey test donee which runs around $25 and they may need the tank 48 hours.
Also, some places are shady and will swap your cylinder for one of their filled ones. That's the scam my welding shop pulled...told me it was a deposit, then call it a sale, and I can't get my full deposit back
Oh, aquariumplants.com also sells
CO2 tubing, as to many other online vendors. its usually not easy to find locally since the diameter we need it's used for many applications...sort of a specialty item.