The New-York Historical Society recently digitized some of its slavery-related collections.
The fourteen collections on this web site are among the most important of these manuscript collections. They consist of diaries, account books, letter books, ships’ logs, indentures, bills of sale, personal papers, and records of institutions. Some of the highlights of these collections include the records of the New York Manumission Society and the African Free School, the diaries and correspondence of English abolitionists Granville Sharp and John Clarkson, the papers of the Boston anti-slavery activist Lysander Spooner, the records of the Massachusetts Anti-Slavery Society, the draft of Charles Sumner’s famous speech The Anti-Slavery Enterprise, and an account book kept by the slave trading firm Bolton, Dickens & Co.
I recommend browsing through the website to see what you can find.