As many researchers have internet access, archives from national to local have put more and more of their archival materials online. It is important to remember that digital archives should never ask you for money and there should not be a paywall for you to access your materials.
A few strategies to get you started with looking for materials around your topic:
Start with looking at the national archives of your topic. For instance, if you are researching the French Revolution then look at the French National Archives.
Get a little narrower in terms of location. Sometimes, local museums will have materials online from a particularly important battle or event that occurred during your time period.
Also look at our National Archives, sometimes we may have materials from all over the world that might be held by the National Archives or even the Smithsonian.
Don't overlook archives at the University level. Many times, donors to universities may have bought historical pieces and those are donated to the University. There may be pockets here and there of documentation related to your topic.
If you are hitting a research roadblock, then rely on the Library of Congress Subject Headings. This source will give you the standardized language and search terms that librarians use for your topic. It should assist with getting more results and also results that align more directly with your topic.