I'm assuming you're thinking of a scenario where your well/city water supply has failed, and you're staying in your home until the situation has returned to normal. If that's the case, you probably already have a 40+ gallon water supply: The hot water heater. (If you have an on-demand hot water system, then of course this isn't a solution.) Assuming the hot water heater is strapped down to prevent it from being damaged during an event, you've got plenty of potable water ready to go.