Digital collections are archival library collections available electronically. Libraries digitize their physical collections and make them available online. These collections can include photographs, manuscripts, maps, books, pamphlets, and audio and visual materials. See below for links to some digital collections.
You can use a search engine (such as Google) to see if archives exist that have materials on your topic.
Once you have found a site (and a collection on that site) , what info on the site (in addition to the digitized item itself) is useful for historical research?