I got my 5lb Catalina aluminum tank off of Amazon. If you have the Prime membership, you can get 2 day shipping for free. If you have a local paintball shop that knows what they're doing, they can fill your tank with no exchanges. As I am part of a large paintball pro shop outfit, I just fill mine off of a 50lb cylinder. People that bring in the tanks for their Kegerators or drink dispensers, we fill them for $15. If anyone on the forum is close to Mandeville, LA, and needs a tank filled, PM me and I'll take care of you on your tanks - 5lb OR Paintball CO2 tanks... (Mandeville is on the northshore of Lake Pontchartrain/New Orleans)
I even figured out how to use a paintball gun regulator to supply CO2 to my 5G Fluval. 20oz tank last me about a month - 5 weeks. If anyone is using a 20oz tank, you should be able to have them filled for around $4.00.