Mostly inter tribal warfare. Slaves were a big source of revenue that made tribal chiefs very rich. They achieved great power over their black people by selling another tribe's black people.

And Arab slavers. The Arabs had a voracious appetite for slaves, particularly since so few of the ones who were caught would survive the journey north across the Sahara and the forced surgical castration. >90% died before getting to the slave markets on the N. African coast. The ones that lived were sold to various wealthy Arabs and later to the Ottomans, though the Ottomans seem to have had a much stronger preference for white slaves out of the Caucasus. The men were funneled into the army. The women were prized in the harems. They'd often command a price 10x higher than the African slaves who were seen as not intelligent enough to be useful as slaves.

But yes, other Africans were probably the biggest source. They'd go in and settle old ethnic scores. They'd raid villages, kill or capture as many as they could, rape the women as much as they thought they could get away with, and then sell them either to the Arabs or the Jewish Europeans on the coasts. Remember when Democrats in Congress famous decided to virtue signal and kneel for 8 min, 46 sec for George Floyd? They wore African cloths to help make the point. The cloth came from a tribe that was made extraordinarily wealthy by, you guessed it, enslaving and selling other Africans who they conquered. =D

It's key to remember that prior to the invention of quinine in the 1860s, Europeans didn't last long in Africa. They'd get malaria or other tropical diseases and die. It was notorious as the "Dark Continent" because European expeditions went in and never returned. They therefore, couldn't map it. On a map, the interior of the continent was dark/unknown. What they had, they got from rumors and stories told to them by Africans and Arabs who claimed to have been there. This is really important, because it makes absolutely clear that Europeans were NOT actively procuring the slaves at all.