Nigeria
The Bight of Benin and the Bight of Biafra supplied a significant amount of slaves to the trade. More specifically in the west of Niger Delta and east of the Niger Delta.
European conquerors initially traded with the inhabitants of the before mentioned, however, they eventually captured them and sold them into slavery. Slaves were acquired via war and invasions.
Other significant ports were in Badagry, the Aro Confederacy and Bonny Island.