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A gallon of the stuff would be from the hardware/paint store or similar chemical supplier. But it probably can absorb 1/2 of it's volume of water and still evaporate off with that, so I don't think you'd need or want a whole gallon. After all, alcohol vapor can be flammable too!
Heck, a heat lamp under the tank should make any water in there evaporate off in a day or so anyhow.
In drugstores you'll usually see 3 grades of isoprop, typically 40-something, 70-something, 95+%, IIRC. Maybe a buck a pint, buck fifty a quart, not very expensive.