Archives and special collections all over the world are working to digitize some of their collections and make them available online. Note that copyright restrictions, privacy laws, and the sheer number of items held in repositories mean that only a very small sample of materials are available online.
One way to search for these collections is by searching online: Use your topic keyword(s) plus the word 'archives'. Also try the words 'papers', 'collections', and 'digital library'.
Searching for historical images on Google or social media might yield results, but it's important to know the correct context, location, and provenance of an image before you use it as a primary source. Anyone can take a historical image and add a caption that tells a fictional but believable story! These trustworthy digital repositories hold large quantities of images and other media.