I'm not sure that I understand the question. The slaves were obtained through trade, and because it was economically beneficial to do so. American colonies were populated largely at first with white indentured laborers from England and its near colonies Scotland and Ireland. If they survived their period of indenture (often seven years), they were free men and often also given land. Some planters in Virginia started importing black slaves relatively early on. I believe at first they were imported from Caribbean sugar colonies, though I could be misremembering, as the sugar colonies had very high mortality rates from overwork and disease (and runaways in the case of Jamaica). Eventually a sophisticated trade network connecting the lumber and agricultural producers (and rum distillers) of the future U.S. Eastern Seaboard, the sugar islands, and the western coast of Africa developed. Slaves were exported from Africa in exchange for rum and other manufactured goods, while the Eastern Seaboard British colonies imported slaves and exported rum (refined from sugar byproducts imported from the West Indies), tobacco, and staple foodstuffs (mostly for the sugar islands, which preferred to devote their arable land to sugar production, rather than food).

You might be interested to read Edmund Morgan's American Slavery, American Freedom, which is one well-regarded analysis of the beginnings of race-based slavery in the Virginia Colony (iirc, he also addresses Maryland and North Carolina, since those colonies were more-or-less the same as Virginia, though less-populated and less-developed).

http://books.wwnorton.com/books/American-Slavery-American-Freedom/