Finding water when your out in the wilderness can be hard at times, but it is doable, unless you are in the middle of an arid zone or in a drought situation. Worst case scenario you could tap a Birch tree to get some water.
Things can be a whole lot different if you are caught in a SHTF situation and you are in the middle of a city or even a run down derelict area, with no streams or puddles you would be hard pushed to find a decent water source, but don't loose faith. There are still a few places where, if luck is on your side you can still get a plentiful supply of water. Yes, you will need to filter it and yes, you should sterilise it and make sure that you boil it, making sure that the water has a good rolling boil for at least one minute or 3 minutes if you are more than 2 km above sea level, but then, this is always a best practice anyway, even if you have set up camp next to a crystal clear flowing stream.
There are at least two places that I can think of which should be able to give you water, even in a derelict house with no running water.
Before we start lets bear in mind the Rule of 3`s, “You Can Only Live 3 days Without Water”, in some situations you may need to take what ever water you can get hold of, regardless. Also bearing in mind that I will be assuming that you will take every precaution to make the water safe before drinking it as we all know that becoming sick from drinking polluted water, especially if you are already suffering the effects of dehydration, will only make matters worse.
The first is a toilet cistern, in older houses where the toilet was installed .before 1991 the average cistern could hole up to 9 litres of water, that's three days drinking water, toilets installed after 1991 hold on average 7.5 litres of water. Of course if you come across a toilet which has a water saving device fitted you should take around 1 litre off the total.
The next place to find water is in the radiators. Unless the building you are in had some kind of ducted heating system you should be able to find a radiator. All you need to do is unscrew the valve at the bottom with a radiator key and the radiator will empty under the force of gravity. It is noted that most places would advise not to drink water from a radiator or house heating systems due to rust and other chemical contaminants and yes the chances are pretty good that the water that comes out will be black and dirty but again, once it has been sterilised, boiled and filtered through charcoal it will still be drinkable.
Other places to consider are park or town square fountains, house and office boilers (which I know comes under heating systems), actual toilet bowls, rain catchers, fog catchers, depending on the time of year you could also tie rags round your feet and walk through long grass to harvest dew.
There are probably lots of other places and ways to get water which I haven't mentioned, the thing is to think outside the box and to put the ideas that a person just does not drink from the toilet bowl or cistern out of your mind. Water is water is life and in times of need, this should be your mind set.