Slaves were traditionally acquired by tribes through warfare—they were prisoners of war, who could be adopted into the tribe, tortured, or set free. Traditionally slavery was a temporary position. American-style chattel slavery—in which a person could be bought or sold, was expected to perform labor, and it was a situation that spanned the person's lifetime—was adopted in the late 18th and early to mid-19th century by assimilated mixed-bloods of various tribes including the Choctaw and Chickasaw.

During the Fur Trade era of the 18th century, Indian slavery was also a profitable business and Europeans and Natives captured Indians and sold them into slavery—often exporting them to the Caribbean where they worked on plantations. The French in particular were active in this slave trade, preying upon Caddo, Chitimacha, and Natchez people in what is now Louisiana.

Seminoles maintained a more traditional practice of slavery in Florida, in which their African slaves paid them tribute (in the form of crops)—the system was closer to serfdom or sharecropping.

It's debated whether African-American slaves fared better among European-American or Native American slave owners. You can read the freed slaves firsthand accounts in the WPA Slave Narratives, Oklahoma and WPA Slave Narratives, Arkansas. Many of these accounts tell of African-Indian ancestors being kidnapped into slavery.

Follow up question- how wealthy would one need to be in this time to own slaves?