Are there any sources of atheists in ancient times, like in ancient Egypt or Greece. If so, how were they treated?

I cannot answer your question for Egypt, but for ancient Greece - yes, there are, although there is some discourse about the extent of 'proper' atheism.

While 'Asebeia', which can be translated with impiety, was a serious offense in ancient greece, that could lead to banishment or even death penalty, some of the accused merely denounced the anthropomorphic character of the greek pantheon, which is per se not yet atheism. Anaxagoras for example stated that for him, the sun was not divine, but a 'glowing rock", which almost got him executed, but then commuted to banishment.

Platon states that atheism was a rather widespread phenomenon at the time, which is a claim Marek Winiarczyk disputes in his book Diagoras of Melos: A Contribution to the History of Ancient Atheism', saying that most of the population would still have followed the traditional gods, only a select few well-educated persons might have had tendencies towards atheism.

In any case, there are a couple of recorded atheists, such as the above mentioned Diagoras of Melos (sentenced to death in Athens, managed to flee), Theodoros Atheos (forced to leave his home of Kyrene, accused of atheism in Athens), Protagoras (banished), or, Diogenes of Synope (lived in the 4th century B.C., when there were significantly less trials for Asebeia, not accused).

Sources: M. Winiarczyk, Diagoras of Melos: A Contribution to the History of Ancient Atheism (available for free on google books), Berlin/Boston 2016

J. Dreßler, Philosophie vs. Religion? Die Asebie-Verfahren gegen Anaxagoras, Protagoras und Sokrates im Athen des fünften Jahrhunderts v. Chr., Norderstedt 2010