Experts say you need at least a gallon of water per day for both drinking and sanitation purposes (the average American uses 80-100 GALLONS per day so you can see how such a sudden change would be a crisis for most people), so going with that estimate, a single person should stockpile 14 gallons for two weeks. Buying three of those five gallon containers of water at the grocery store would work. You can also buy fourteen gallon jugs of water at the grocery store (the containers are usually pretty flimsy though), or a bunch of cases of bottled water. If you are feeling especially flush with cash you can even buy canned water which has a pretty long shelf life. Any of these will work as long as you end up with a nice stockpile of water.
After you have done this, plan for other sources of emergency water including:
- the water in your hot water tank.
- filling up the bathtubs with water ahead of an emergency.
- catching rainwater.
- a local creek/lake/stream/river.
- abandoned wells/hand dug wells.
- cisterns/seeps/natural springs.
You will also need a process for purifying the water you find so have multiple methods for this on hand as well (purification tablets, boiling, backpacker's water filter system, etc).