As usual, your surroundings and what you have around you will determine what method you use to provide yourself with water. Sure, advanced preppers inform that in extreme conditions, some water sources could be unusual ones, such as toilet tanks, water heaters, irrigation systems, the grid pipelines itself in their lower spots, and others.

Even fire hydrants could be a water source, but (and it's a BIG but) these systems are under a VERY dangerous level of pressure, and I would advise against it unless you know very well what you're doing. Furthermore, the chances are that this water is chemically treated with products to avoid corrosion, for example, and unsafe for our purposes. So, buyer beware.

Getting water when there is no power after a city-wide collapse is difficult.

My point is that cities are dangerous places to live after a collapse. Consider the rise in crime, for example.

Urbanites are definitely at a disadvantage here when we talk about having access to safe water continuously. Let's say the grid stops working. If your big city job is gone (like what happened to me), then staying in the city then is probably not your best move. We can identify basically three water sources that should be safe once SHTF and we need water.