+1 on the 2 liter pop bottles. I once was an apartment dweller and these filled their purpose of water storage splendidly. There was a little cubby hole under my stairs and I put about 10 2 liter pop bottles in there, stored with water. As long as they are stored in a cool dry place and you check them once in a while it should be okay.

If you're worried about dropping them/leakage you can do what I did and carry about four bottles in a milk crate with handles for easier transport if need be.