Libraries submit records for archival documents and collections to Williams WorldCat. After searching look for these formats in your results:
Limiting to these formats may bring up a freely available digitized document, an online collection guide, or a link to a subscription database. For subscription databases, Williams must have a subscription for you to access the materials.
Maryland and Slavery (search includes all formats, look at list to see variety)
Libraries, historical societies, and database vendors are digitizing archival materials to make them more accessible. Below are just a few such projects.