I have a lot of commercially bottled water, but if I were to do it myself, I'd use tap water and then add a drop of unscented chlorine bleach to it. Also, you can drink the water from the toilet CISTERN (not the bowl, unless you're a dog). Also, in the event of a water stoppage that doesn't require evacuation, there is a LOT of water left in the pipes if you go to the lowest part of your house and open a tap. Also, they make ceramic-disc filters (for use while camping, for instance) that filter out EVERYTHING bad for you, even protozoa. All you do is use a reasonably clean body of water (like a river, but not like a mud puddle) and pump it into cups or a jug.