Question

How did the West African slave traders obtain the slaves they sold to Europeans?

Answer 1

Trough tribes battles.

Explanation:

Africa was(and still is in a lot of places) divided by tribes. Each tribe would fight with other for resources, women, land and for other reasons. The Western Africa was already colonized, most of the countries by Portugal and Spain, and they had business and agreements with the natives, and since those countries had way more resources than the African countries/tribes, Portugal and Spain would "possess" the coast and some other territories through might.

This way, the tribes that won the fights and battles would capture the people of the other tribe and sell or trade them with the Europeans to get some artifacts(mirrors are a good example) and some resources.