Most local libraries have a genealogy room or collection of some type related to local genealogy. These collections should not be underestimated. For example, the federal census records are taken every ten years. Did you know that some local jurisdictions take census information annually? Having access to this information can fill in the gaps between census recordings. You might actually be able to pinpoint the exact year of changes in family status or occupation or property ownership. Other records include lost cemeteries, property tax records (including slaves), schools, founders of the town, business records, newspaper clippings and maps of the area.
Other notable collections include family histories donated by local researchers. This is important to your research because you can extract information about relationships from these histories. For example: let's say that genealogist John Smith writes that his great-great grandfather owned 16 slaves. He includes their names, birth date and their children in his research. He also includes his great-great grandfather's Will which allows the sale of six of those slaves to his sons and daughters for one dollar each, one of which is your great-grandmother. A natural question to ask is why did he sell six slaves to his children rather than just give the slaves away? Was it to make the purchase easy on his children or was it because those six slaves were also his children? The answer is most likely the latter.
The last Wills and Testaments for infamous ("infamous" is an appropriate adjective for enslavers) people can be found online, at the Library of Congress or the National Archives. However, it's the local slave owners Wills that provide the most helpful information in your research and they can be found either at the local library or the local Courthouse.