Anyone looking for primary source materials housed in archives across the world is in luck, as the website Repositories of Primary Sources will be your free research matchmaker! The site lists 5000+ websites which describe “holdings of manuscripts, archives, rare books, historical photographs, and other primary sources for the research scholar.”
The repositories are divided by geographical location — as far as I could see there is no search function. This project is really useful and the fact that it doesn’t cost anything to use is much appreciated. Other sites like Archive Finder require a subscription.