Hey Ryan, I rent my tank from a place in Milton called Unitec. They are a welding supply company. They rent a 20 lb cylinder for 20 dollars a month, costs 30 dollars to fill it. They do not have tanks any smaller than 20lbs. I run three tanks with it with a three way manifold. It usually lasts a good 6-8 months running 24 hours a day. I am going to buy a tank from aquariumplants.com soon, just been to lazy to do it!
They will sell you a 5lb tank empty for 55 bucks us, a 10 lb will run you 75 bucks empty. They will sell them full as well, just add another 30 bucks to each tank.
I would check out Unitec, there is probably one in your area. Only catch with them is they won't fill the bottle for you unless you rent it from them. If you bring your own bottle to them they won't fill it. I am in the process of sourcing out another supplier of C02 and am going to buy a tank shortly, just been to lazy!