"Ever since the day that the Temple was destroyed, prophesy has been taken from the prophets and given to fools and children." (Babylonian Talmud, Bava Bathra 12b)
The Talmud passage then goes on to give examples of rabbis receiving prophesies from a lunatic and a child.
Another example from Zohar:
Rabbi Hiyya said, "Thirty days before a nation rises to power or is taken by calamity on earth, that event is proclaimed to the world. Sometimes it is proclaimed by the mouths of children, sometimes through people who have no sense, and sometimes that word is transmitted through the mouths of birds, who announce it to the world, yet no one notices." (Zohar II 6b)
This Zohar passage then goes on to give accounts of rabbis receiving prophetic messages from a bird as it flies in the sky and from passing children.
A recent example of this belief in practice was documented at this blog at the following link:
Kabbalists: Great, Miraculous War Coming