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A lot of primary sources are published in books and you can use library catalogs like HoyaSearch to locate them.
The Library of Congress tags every book that is published in the US with subject headings. There are several subject headings that are used for primary source materials, such as
Description and travel
A more complete explanation is listed here. When you do a search in HoyaSearch (or another library catalog like WorldCat), try using one of these terms as a SUBJECT search and then adding in a keyword. Your keywords might have to be pretty broad, as you are only searching a very small amount of information about the books - NOT the full text of them.
If you find a book that is relevant for your topic, make sure you look at the subject headings by clicking on the Details tab on the left-hand side in HoyaSearch. This will give you other good search terms. The Library of Congress often uses strange vocabulary and you'll get better results if you can use it effectively.
A searchable collection of finding aids to primary source material held in archives, special collections, and manuscript collections around the world. Includes historical documents, personal papers, family histories, and more.