Do your absolute best to find other related ancestors who were not enslaved; this may help you track down enslaved ancestors. One option is to start in the present and work your way backward. This will help you narrow your search for an ancestor that was enslaved.
Be compassionate with yourself on this journey to find your ancestors. It is possible that this will be a difficult search, one that not only presents practical obstacles, but also constantly reveals the horrors of chattel slavery. Remember your own humanity, and look for signs of your ancestors' humanity (their names, their relationships, signs of resistance and resilience), even if the records you are finding are inherently dehumanizing.
Be prepared to center your search around the enslaver, not the enslaved. Due to the types of records that were maintained and deemed important (property deeds, will and testaments, account books), and due to the fact that many archives collected the papers of plantation owners and enslavers in order to enshrine and sometimes glorify prominent Southern figures, slavery, and the Confederacy, many of the records that are contained in archives were created by and for enslavers. These records are often dehumanizing and incomplete. So, you will probably need to search for the name of the enslaver or the plantation, and then see if you can find documents that name enslaved people.
If, for instance, you know that you had relatives in Durham County who had the last name of Cameron or Bennehan, it is possible that they may be descendants of enslaved people on the Stagville Plantation (which was owned by the Bennehan and Cameron families). You could begin your search in the catalog or in the collection guides by typing in "Stagville."
While you may need to center your search in the catalog records and collection guides around the names of the enslavers, there are other resources that focus on the humanity of the enslaved peoples. For example, the Enslaved Peoples Database (the result of a collaboration between many libraries, archives, and researchers) allows researchers to search for the names and occupations of enslaved people from all around the world.