most stores that carry hunting/camping equipment should carry water storage containers. I bought a 20g blue one at Sportsman's Wherehouse not too long ago. I had to cut out the hole to fit a MJ in there but it's perfect. Cost about 20bux. Other than that I'd call water stores and see if they have any other containers... the type of store where you can fill up your own 5g jugs, they might have bigger ones.
OR you could go buy a 32g BRUTE container from Lowes, perfectly reef safe, holds more water, only costs about 20-30 bux depending on where you get it, and you can buy rollers for them so you can wheel it from your ro/di unit to your tank. hth