You may be looking for an old family recipe for Snickerdoodles and you ask your great-aunt who has the recipe memorized. A second kind of primary source is observation. Your research purpose could be to find out if squirrels like banana leaves and then you could set up an observation spot near a banana tree and see if squirrels eat the leaves. But, a word of advice. Don't bother checking this one out.
Squirrels do like banana tree leaves.
They've eaten mine down to the nubbins. A third kind of primary source is a questionnaire or poll. These are simple to do and easy to obtain the results. You can see an example of a poll on this site. Another kind of primary source is to do an interview.