Don't forget that most homes have a built-in 40 gallon (or larger) water reservior - the water heater. There's also several gallons that can be harvested from the pipes. Just open a faucet at the high end of the system and collect water from a faucet at the bottom end.

In cases where the municipal water is still flowing, but becomes contaminated, it's important to remember to turn off the water main to your house first so you don't get contaminated water in your system.

Granted, it may not last all that long, but it could bridge the gap during a dry spell or something like that.