Source for RO pressure tank

I've got a 5 year old 5 stage RO system. I'd like to start using RO water to make beer but the 3.2 gallon tank takes two days to deliver the 5 gallons of purified water necessary for a single batch of beer. That's assuming I don't try to make a pitcher of iced tea while I'm waiting, which can add yet another day. Needless to say, I need to make a change. Would a 14 gallon tank allow me to draw 5 gallons of purified water at one time? FWIW, I have a 100gpd TFC membrane, which I assume is up to the task. If so what's a good source for pressure tanks at reasonable prices? Online is fine, but what kind of brick-and-mortar stores sell parts like this?
Other suggestions?
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Mike Hartigan wrote:

How about two samll ones in tandem?
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