Here's a new source, or rather a new way to get access to an older source. Betty Wood put me on to it the other day with a great paper she presented here at Cornell.
The source is the "Book of Negroes," a roster of African and African-American people who left New York with the British in 1783 on "Evacuation Day." Many, if not most of them ended up in Canada. Some of the entries give glimpses of their lives before Evacuation Day: names of previous enslavers, where they came from, etc.