The answer is, from the natural water cycle. This is a continuous process which sees clouds in the sky being formed by evaporation of water from the land, seas, rivers and lakes.
The water in the clouds falls as rain when weather conditions dictate, finding its way back into the ground, our rivers and the sea – and so the cycle starts again, as it has done for millions of years.
Rainfall eventually finds its way to the nearest reservoir or river, or percolates through porous ground such as chalk into aquifers. We then collect and treat this water, before sending it your home for your essential daily needs, such as drinking, washing and cooking.
The resulting waste water we all produce is then fully treated and cleaned by the water and sewerage companies before it is returned to the environment.
At every stage of its route back to the sea or rivers, some of this water evaporates to begin the water cycle once again.