Water

There are water purifiers that exist that allow you to filter your own urine into clean drinking water. Hopefully your situation won't come to that.

Well water
The best source for clean safe drinking water is from an unpolluted well. Those in your community who tap this source, should be required by local law to make sure it isn't an open well someone can toss garbage into, but instead properly covered. An electric pump can be used, with a hand powered pump for a backup as well. This water source will not require any filtering at all.

On January 23 2019 a study was published showing that microplastics had contaminated a lot of groundwater. Other things can get there as well. In a doomsday situation, it is possible a lot more pollutants will be about and can cause contamination.

Pond or lake
Using an existing body of freshwater, or making your own with borehole explosions or other methods, allows you to have a supply of water to tap.

Rain barrels
Collecting water from a roof into a rain barrel, allows you to have a renewable resource of water. You should be prepared for the possibility of drought, and either store up enough water to be safe, or find another source.

River, stream, or canal
Concerns about pollution, blockage, and hostiles using it for transport and attacking you, should be addressed. Can you bury a pipe to tap the water without anyone noticing it?

Ocean
Various methods exists to heat the saltwater from the ocean until the water evaporates, leaving the salt behind. The vapor is collected and turns into fresh drinking water. A cheap electric distiller can be found for under a hundred dollars which is used to filter tap water. It can be used to turn saltwater into freshwater as well.

Things with water in them
Urine, a container of cola, anything imaginable, can have water filtered out of it.

Water tower
A water tower allows you to store water, and use gravity to give you proper plumbing facilities.

Filtering


Water filters are so advanced now, you can turn Coca Cola into pure clean drinking water.

Plants to purify water
The aquatic moss Warnstofia fluitans can remove 80% of the arsenic pollution in water in an hour.