You need 21 gallons per person for a two week supply.
Don’t forget to include bleach in your supply for purification.
Large gallon barrels, more expensive option, price ranges depending on supplier.
Water bottle cases $3.99/24 (or cheaper...watch for sales!)
Mylar bags $3.49/1 holds 5 gallons (http://www.beprepared.com/)
Water pouches (4-8 oz) $.35/1 (http://www.beprepared.com/)
I always try to buy a case of water when I make a big trip to the grocery store, or at a warehouse store like Sam's Club or Costco. Then I rotate this water out. Don't forget about the five gallon jugs for your water cooler. Those count towards your supply!