www.aquariumplants.com has an empty 5lb aluminum canister shipped (to me in MI) for about $66. That's not a bad deal at all and you most likely won't find better. When I was looking at cansiters several months ago I think the cheapest I found was in the low $70's including the link mentioned.
Do the math. I found that buying a 'cheaper' tank online, having it shipped, and then having it filled was the same price as just buying a filled one from my local Robert's Oxygen.
YMMV - you just might want to check it all out. Refills alone cost me $21
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~Neilan
In the DC Metro Area? Check out GWAPA and WAMAS