Water tanks can be found adjacent to builings in some cities that have areas of low water pressure and in rural areas without hydrants for fire sprinkler systems. Codes require the tanks to be full. Tanks that are 50-100 thousand gallons are common. Most of the vavles are locked so a pair of bolt cutters would be necessary.
After an EMP event this water will be unobtainable after a hard freeze. When it finally thaws the pipes will be broken.
Volunteer fire companies usually have a tanker truck full of water that might be useful if it’s not being declared by the members or town.