from: http://www.wissen-erleben.de/roots/wherefrom.htm

History of Slavery
Where did the slaves come from?

The majority of African slaves shipped to America came from Gambia, the Gold Coast, Guinea, Sierra Leone, Upper Volta, Ivory Coast, Ghana, Togo, Benin, Nigeria, Cameroon, Gabon, the Republic of the Congo, and Senegal.

At the beginning of slavery the slaves were taken from the regions close of the coast, where smaller forts were established. But gradually the coast was empty and the population of the forests was strongly decimated.
So the African kingdoms, which operated the trade with the slave hunters, forced their way deeper and deeper into the inside Africa, in order to catch more prisoners. These prisoners where caught in the way in which animals are catched. Because for the white ones the Africans were not humans.
After this hunt the prisoners had to begin a long way of suffering. If e.g. this way up to the coast amounted to thousand kilometers, each second prisoner did not arrive at the ship. They were underfed, with stick impacts badly treatened and at each resistance strucked down or killed immediately. This way alone brought the death of eight million humans.

The following table shows the number of Africans brought to America from 16th to 19th century:

16th century:

600,000

17th century: 2,000,000
18th century: 5,000,000
19th century: 3,000,000