You're right that many wars fought by Greeks were fought against other Greeks, particularly in the Archaic and Classical periods. You're also right that, as per the 'rules of war' (not so much rules as modern rules, more like a cultural way of doing things that need not always be adhered to), defeated enemies were largely at the complete mercy of the victor.

Take, for example, this description of what happened to defeated cities in the poetic world of Homer:

And then at last his wife, the fair-girdled bride, supplicatedMeleagros in tears, and rehearsed in their numbers before himall the sorrows that come to men when their city is taken:they kill the men, and the fire leaves the city in ashes,and strangers lead the children away and the deep-girdled women

Iliad 9.590-9 (trans. Lattimore)

Moreover, we see this in practice during the Peloponnesian War. After the Mytilenean Revolt, slavery was a potential punishment for the population of the city (Thuc. 3.28), and the fall of Plataea saw the execution of the Plataean and Athenian men defending the town, while the women were enslaved (Thuc. 3.68). The most infamous episode of such behaviour is the aftermath of the Siege of Melos, when the entire male population was massacred and the women and children sold into slavery (5.116). Indeed, so memorable was this occassion that the Athenians feared the same fate awaited them after their defeat by the Spartans at the end of the Peloponnesian War (Xen. Hell. 2.2.3). However, the Spartans refused the call of their allies to enslave Athens (Xen. Hell. 2.2.20). It might have been the case that the massacre of the entire male population of a town was actually not that common, and the occassions that were recorded were done so because of the severity of the conclusion, particularly if we consider the emphasis Thucydides places upon these events in his wider narrative. According to Braund, "When cities fell, there was a recurring tendency for the victor (even when dealing with Greeks) to kill the men and enslave the women and children." However, "the lack of Greek slaves in the historical record for classical Greece suggests that Greek victors usually preferred another course" (Braun, 2011: 116).

One option available to Greeks who were enslaved in war or raiding was ransom. Demosthenes tells us of an Athenian man who was enslaved on Leucas, a Greek island in the Ionian sea, west of mainland Greece, who was ransomed for 26 minae (Dem. 57.18-9). However, given that 26 minae is "roughly ten times the average price of a slave" (Braund, 2011: 118), and the average price of a slave was potentially half a year's wages for the average Athenian, then ransom may have been restricted to wealthy individuals, and was only an option so long as an enslaved individual had access to their wealth.

So far this looks like slaves would likely have been Greeks. However, this doesn't take into account the near constant slave supply form beyond the Greek world that was created from raiding and trading. Even during campaigns in large wars, Greeks might turn to some opportunistic raiding. The force of the Athenian general Thrasyllus, while on a campaign against the Greeks of Asia Minor, made a raid against the Lydians, seizing most notably money and slaves (Xen. Hell. 1.2.4). Additionally, during the march of the 10,000, the army actually makes a detour to procure slaves (Xen. Anab. 6.6.38). Such raiding was so ubiquitous in Homeric (and possibly Archaic) Greece that Moses Finley could go so far as to say that a "typical [Homeric] war" was one of raids for plunder (1977: 46). Of the many raids mentioned in the Homeric poems, there are several that involve raiding non-Greeks. Odysseus' raid on the Cicones of Ismaros takes women slaves (Od. 9.39-43), twice Odysseus tells of raids on Egypt (Od. 14.257-65, 17.223-43), and Herodotus also notes Greeks raiding the cost of Egypt (Hdt. 2.152), and the Taphians are said to have raided the area around Sidon to capture slaves in a manner that resembles a potential Greek raid on the same area from the eighth century (on the Taphians' raid see Od. 15.426-7; on the Greek raid see Parker, 2000). Thus, Greeks actively went raiding for slaves in the Archaic period. This does not mean that raiding was not conducted upon other Greeks though. Such piratical activity was fairly common, and there was even a Teian law forbidding the harbouring of traders during the Classical period, suggesting they were active in the Aegean (Meiggs and Lewis, no. 30).

Besides raids, Greeks would also likely have taken barbarian slaves in the course of their colonial expansion. Archilochus' poetry records the conflict between the Greek colony of Thasos, "thrice-wretched city" (fr. 228; cf. fr. 102), and the local Thracian tribes, with Archilochus even saying that he lost his shield when fleeing from them (fr. 5). Such conflict between native inhabitants and new-comer colonists was likely rather common, with captives of such wars being taken as slaves and sold. The Cyllyrians are a potential extreme case of such enslavement of locals, being the douloi of the gamoroi of Syracuse (Hdt. 7.155). While we don't know much about the Cyllyrians, with what Herodotus tells us being pretty much it, it is assumed by scholars that they were Sicels, natives of Sicily, enslaved by the Greeks. Although De Angelis has suggested that they might have been dependents of the local, native Sicel elite, and came to be dependents of the Greeks after the absorbtion of the local elites into Syracusan society, rather than forcibly enslaved (2016: 164). Rihll believes the expansion of the Greek world with the establishment of colonies might have facilitated the growth of the Greek slave trade (1996: 102-4), particularly as the acquisition of slaves was a relatively low cost endeavour (see Aritoph. Wealth 522-4; Xen. Symp. 4.36). Braund has actually suggested that the ease of acquisition of barbarian slaves actually facilitated continued Greek exapansion (2011: 123). Either way, slavery was certainly a significant factor in the development of Greek overseas settlements.

Even after the 'golden age' of Greek colonisation in the Archaic period, Greeks continued to rely upon barbarians for their slave supply. Polybius tells us that slaves were among the necessities brought from the Black Sea region (4.38), and Strabo tells us that the nomadic peoples about that region provided slaves, among other things (11.2.3). Thus, it can be assumed that, in exchange for goods, such as wine, the Scythians and other tribes of the Black Sea region acquired slaves and traded them with Greeks, establishing a trading network between mainland Greece, the Black Sea colonies, and the barbarians. A similar trade network existed with the Persian satrapies of Asia Minor, particularly Phrygia (see Lewis, 2011).

Therefore, while slaves were certainly a product of war between Greeks, Greek-on-Greek warfare was not the primary source of slaves, particularly in the Classical period. Instead, conflict with barbarians, as well as networks of trade with them, facilitated the movement of slaves, among many other goods, throughout the Mediterranean.

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Bibliography

D. Braund, 'The slave supply in Classical Greece', in K. Bradley and P. Cartledge (eds.) The Cambridge World History of Slavery, vol. 1: The Ancient Mediterranean World (Cambridge, 2011), 112-33.

F. De Angelis, Archaic and Classical Greek Sicily (Oxford, 2016).

M. Finley, The World of Odysseus. 2nd Ed. (1977).

D. Lewis, 'Near Eastern slaves in classical Attica and the slave trade with Persian territories', Classical Quarterly, vol. 61 (2011), 91-113.

B.J. Parker, 'The earliest known references to the 'Ionians' in Cuneiform sources', The Ancient History Bulletin, vol. 14 (2000), 69-77.

T. Rihll, 'The origins and establishment of Greek slavery', in M.L. Bush (ed.) Serfdom and Slavery: Studies in Legal Bondage (London, 1996), 89-111.