Bread can often be found not only at bakeries, but at other places, too, since many shops selling bread tend to ridiculously overstock, that's why bread (often in alarming quantities) can be found in many dumpsters.
Often you could take large quantities of bread, but if you do you find yourself throwing away the bread again. Especially beginning dumpster divers can feel tempted to take all the bread they find - and then find themselves throwing away most of it 1 or 2 days later. A good idea is to freeze what you think you will not use very soon, or to wrap the bread in a plastic or linen bag, and keep it in a dry cool place.
There is a rule in both Judaism and the Islam that people should not throw away food. In Jerusalem you can find random bags of bread right in the street.