"Thousands"??
I highly doubt you can find credible documentation to support that.
Why do you doubt that? You could find the information if you wanted to.
According to federal census reports, on June 1, 1860 there were nearly 4.5 million Negroes in the United States, with fewer than four million of them living in the southern slaveholding states. Of the blacks residing in the South, 261,988 were not slaves. Of this number, 10,689 lived in New Orleans. The country's leading African American historian, Duke University professor John Hope Franklin, records that in New Orleans over 3,000 free Negroes owned slaves, or 28 percent of the free Negroes in that city.
In 1860 there were at least six Negroes in Louisiana who owned 65 or more slaves The largest number, 152 slaves, were owned by the widow C. Richards and her son P.C. Richards, who owned a large sugar cane plantation. Another Negro slave magnate in Louisiana, with over 100 slaves, was Antoine Dubuclet, a sugar planter whose estate was valued at (in 1860 dollars) $264,000 (3). That year, the mean wealth of southern white men was $3,978 (4). http://americancivilwar.com/authors/black_slaveowners.htm
I should have added - some blacks actually were slave breeders who sold their own children into slavery.
Also, apparently after the Civil War some blacks petitioned to go back into slavery because they felt they couldn’t care for themselves and were afraid they would be sent back to Africa.
This information is available if you look for it. Obviously you will not find it in the New York Times or talked about on CNN.