Combine keywords of the type of format with your topic. It may look like this:
"nuclear arms" pamphlets
clam worm methodology
slavery "personal narratives"
Lyndon Johnson diaries
Ancient greek military maps
"World War II" posters
Other primary source words to try adding are:
cartoons or caricatures
Limiters will let you get specific about what EXACTLY you are looking for.
Select check-boxes like this:
And some will even have Primary Sources separated out: