No single website can offer a comprehensive search of all primary sources. Instead, use specialized research tools to identify relevant primary source documents.
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Search strategy: use bibliographies to identify primary sources
Start by using Tripod to find a recently published scholarly book related to your research interests, written by an historian. The bibliography will likely separate the author's sources by type.
For example, look at this bibliography from Our Separate Ways: Women and the Black Freedom Movement in Durham, North Carolina. The author includes a list of archives she visited and periodicals (e.g. magazines, organizational newsletters, newspapers), among other sources.
So by finding a scholarly book related to your topic, you might be able to identify a specific newspaper or magazine, an online database, or an archive - related to your research interests - that you could visit or use.