There are many sources for finding manuscript and archival music material, including composer's archives and papers (meant here in the sense of correspondence, sketches, and other unpublished materials). These include, but are not limited to:
See the other boxes on this page for links to some of these tools.
Many libraries have begun digitizing their primary source materials. Once you know which library owns a specific manuscript or edition, check the library's website to see if a digital image is available.
The following sites give access to major research libraries: