This page of the Mr. Prepper guide explains where to get water.
Water is practically a basic element of Mr. Prepper gameplay. It is used to make soups, silage jars and watering plants in the garden. At the beginning of the game, water is acquired mainly from the sink. You can't disassemble it - you can have only one sink. You can retrieve water only when the icon is blue, which means that the container is full. You take 5 units of water at a time. It takes approximately 24 hours to refill the container.
The second way of obtaining water is using buckets - you can craft them on Level 2 of Workbench using 1 Metal and 1 Plastic. The number of buckets you can craft is unlimited. Put them outside and wait for the rain. 1 bucket grants you 1 unit of water. Keep in mind, however, that the buckets are not fully legal- you can only have 2 buckets in plain sight, the rest must be hidden from the inspection agent.
The last way to obtain water is Waterloo 1815, which you can build only on Level 4 of Workbench. You can install this device in your basement and retrieve water from it every short period of time (1 unit per each use). This device requires electricity, but allows you to become independent of external water supply (only to a certain extent).
In the later stages of the game, there will be days with no water supply from the authorities. Therefore, it is recommended to collect water as soon as the sink is full and storing it in cabinets. Later on, tap water usage will be monitored and inspected.