The question is vague, as it is not asking about a specific period. It is assumed by the question to be related to American Slavery, as slavery of many sorts has existed from biblical times.
Most of Europe had outlawed slavery, except Portugal. They still conducted raids with the help of natives, and the implicit consent of England. The reason for the large influx of slaves in America came from trying to grow rice in the then swamplands of South Carolina. malaria was killing off white settlers at rates approaching 90%. Africans had some immunity and they also had many carriers of the sickle cell gene, which when only a carrier, does not cause the disease and makes a person's blood immune to Malaria.
Where did the African Americans go? It amazes me but today there is still the feeling of pride by those who came from the West Indies, or trace their ancestors back to slaves from the West Indies. This was the port stop off. Those thought to be suited to working in home would be educated, taught refinements, and so on.
Many never survived the trip and just died in the ships, were killed by being thrown overboard if food was low, if trolling ships that disallowed slavery were around, and so on. Presuming the person survived, and was not grabbed by the West Indies, their lives depended on their "Owners".
It needs to always be remembered that even after the Emancipation Proclamation, society would not accept many ethnic groups and certainly the African Americans were part of that. Paul Robeson, an outspoken man who traveled the world trying to enlighten people about the plight of "Negros" in America, would not live here, the son of a freed slave himself. He returned in 1939 and after only a short period of success was silenced by the House on Un-American Activities Committee, so performed as a slave until given back his passport in 1958.
to the southern states.
Brazil
BRAZIL
Many of the slaves left to go to Canada.
on a boat to America
Canada
Most slaves in the country usually lived in the southern part of the United States.
Nigeria
Canada
Portugal
According to the article at the related link below, there are 14,000,000 slaves in India.
Brazil
BRAZIL
Africa To improve upon the above answer. Africa is not a country but a very large continent. I do not know the first country where slaves came from, but most were from western or central Africa. Most probably came from Nigeria.
Portugal, I think. Portugal was definately the country that traded the most slaves
Many of the slaves left to go to Canada.
on a boat to America
Spain