The digital collection at Beinecke Library (Yale University) had quite a few books and original documents from colonial America through the mid-19th C. on slavery. Just enter slavery into the search box at the end of the page Digital Collections at the Beinecke Library | Beinecke Rare Book & Manuscript Library (yale.edu).
The Gilder Lehrman Center for the Study of Slavery, Resistance, & Abolition at Yale may also have some helpful documents and publications. Online Resources: Slavery and Freedom in New England | The Gilder Lehrman Center for the Study of Slavery, Resistance, and Abolition (yale.edu)
Dear Ms. Gailor,
Thank you for posting your request on History Hub!
We suggest that you contact the Massachusetts Archives, the New Hampshire Archives & Records; the Rhode Island State Archives; and the Connecticut State Library for any records pertaining to the church and slavery that may be part of their colonial records.
We also suggest that you contact the Congregational Library & Archives for any records they may have on the Congregational Church and slavery in colonial New England.
We hope this is helpful. Best of luck with your research!