You may be interested in the story of Epictetus. Epictetus was born a slave in Hierapolis, which was in southern Anatolia. Since Greece was under Roman occupation, he spent his time as a slave in Rome, serving Nero's secretary. He was very interested in Stoicism and studied philosophy passionately. He eventually gained his freedom and one of his pupils named Arrian compiled his teachings into the famous Discourses. It's argued whether Discourses are actually from Epictetus or original works from Arrian (kind of like the Plato/Socrates relationship).

The work is fragmented but it's some of the best stuff we have that was written by a slave. Again, we cannot be sure though whether Epictetus was the original creator of the Discourses.

It has been a few years since I read some books on ancient Greece, so I apologize about my lack of info. IIRC many slaves were Thracian and Scythian, among others. The Thracians were a (according to Xenophanes) very large group of tribes of people with red hair and blue eyes. Their lands were directly north of Greece. The Scythians were north of Thrace and were a nomadic Iranian people. After a battle winners often took the surviving opposing force into slavery. My understanding is this practice is what led to many becoming slaves. The Scythians were not Persian per se, but they were of the same Iranian peoples.

From what I have read, most slaves were other Greeks. Spartans enslaved tens of thousands of Southern Greeks during their conquests of the Peloponese.

The vast majority of Ancient Greek Wars was between Greek city-states themselves, as in Greece was basically in a state of what we could consider Civil War non-stop for almost a thousand years with short breaks to defeat the Persians.

Usually slaves were taken after a battle or conquest so Greeks would just take each other after each victory