More specifically my question is how were African slaves acquired in Africa? Were they hunted/captured by European slave trading expeditions, or were they bought or traded in some fashion from other African tribes or individuals higher in the social hierarchy?
Thank you in advance for any insight!
Most of the time, they were bought directly from other Africans. The idea that slaves were kidnapped and captured by marauding Europeans is largely a myth, and one of the most harmful ones that persists. Africans owned slaves much like Europeans owned land so going into their lands and stealing their property bode ill for their relations, thus it was a rare occurrence. Most of the slaves, about 80%, were bought directly from Africans. 1493 by Charles C Mann has some great info on the subject of you want a more in depth look into the matter.
A few slaves would be captured or enslaved from the coastal regions, but usually they were from inland locations and captured by the Coastal African tribes or by African-European slave dealers. Some slaves were specifically captured to be slaves, most others were caused by the constant warring between different tribes.
Some Historians believe that a lot of these inter tribal wars were manufactured by the Europeans specifically to create more slaves. In Slave Trade Debates 1806, Colonial History Series, Dawsons of Pall Mall, London 1968, pp. 203-204. we see the quote "All the old writers... concur in stating not only that wars are entered into for the sole purpose of making slaves, but that they are fomented by Europeans, with a view to that object." This lends credence to the idea that it wasn't just Europeans capturing Africans, but Africans enslaving Africans.
This is an excellent book on the subject if you want to do some further reading, Thornton, John: Africa and Africans in the Making of the Atlantic World, 1400–1800, 2nd edn Cambridge University Press, 1998.
Edit: Submitted too early on accident, left out a whole lot of info.
TL;DR Mostly Africans selling Africans.