It sounds like you want slave narratives. We have them in some abundance and as they were mostly published in the 19th century, they're free. Most of them are reasonably short, though you have a few book-length works. Titles often say where they were enslaved.

These are firsthand accounts of enslaved people, but it's worth going in with some caution. I don't think any of them was written while the author was still enslaved. Rather they're retrospective and written with an audience of white abolitionists in mind. Many of them probably bear a fair helping of white editorial work on top of that. Literate slaves were rare, so the standard scenario is probably closer to a white person interviewing and writing things down than the enslaved doing it themselves. They are problematic, but not much more so than any other sources and we are unlikely to ever turn up anything closer to the enslaved speaking for themselves without a filter than what we have now.

That said, I would be much more careful with narratives from the WPA. They are products of the Jim Crow South, where white interviewers who may literally be the children and grandchildren of enslavers ask former slaves who mostly only knew slavery as children or teens just how bad it was. Even if they didn't make it explicit just what kind of answer they wanted (and it appears they often did that) one can't really expect full candor.