There's an archivist named Alan Lomax who collected recordings of old songs and put them in collections. There's too many to name in a ton of different styles. Just look up "Alan Lomax Collection" and you'll find them. It's all that kind of old timey folk music. Prison Songs is one of my favorites. It's recordings of songs sung by prisoners on the old chain gangs from the Mississippi State Penitentiary back in the mid 1900's. Definitely has O Brother, Where Art Thou? vibes.