In addition to using the resources on this guide, think about who is likely to have sources pertaining to your topic. Is it a topic of particular relevance to a geographical area? Look for state and regional historical societies online. Start HERE
or google to find them. Universities in an area may also be a source to try.
Sometimes they'll have digital repositories of primary source documents, or sometimes you can find a human being listed that you can contact - a librarian or archivist who might be willing to work with you from a distance. If you're able to travel to the site, you can do a lot of preliminary research ahead of time, so that your time in that library or society is well-used.