A manuscript collection is comprised of the hand-written or type-written work of an individual who lived during the period of history under study. Frequently, manuscript collections of famous people and organizations are preserved in libraries and research centers. Manuscript collections are generated naturally through the life of the person or organization which created them. Frequently, scholars use these collections (often, when they belong to one person, they are called papers) as primary sources for research.
Most north American libraries list their manuscript collections through the world's largest library catalog, WorldCat. Think of WorldCat as a library catalog that holds the bibliographic information for collections at libraries all around the world. You can search WorldCat to find the location of manuscript collections which might be useful in your research project. A collection may or may not be digitized. WorldCat's listing should make it clear whether you have digital access.
A tip: manuscript collections of individuals might also be called their papers, essays, correspondence, letters, diaries, or any other type of document which might have been generated by that person. Organizations might produce records, ledgers, or other types of documents.