It depends on how many balloons you're blowing up. The small tanks (party size) that are "disposable" blow up something like 25 balloons. You can get them at BJ's or similar stores, usually a little cheaper than a party store (about $20). If you're blowing up more balloons (hundreds?), you can usually rent a commercial size tank at a party store - when I've gotten them it's like $50 plus a significant deposit so they get their valve back.