The "primaryness" of a source emerges in relation to the researcher's engagement with it. The UC Berkeley Libraries define primary sources as "either created during the time period being studied or...created at a later date by a participant in the events being studied (as in the case of memoirs)."
(source: Finding Historical Primary Sources: Getting Started, last updated 8/12/18, http://guides.lib.berkeley.edu/c.php?g=4409&p=15606)
When determining primary sources for your research, it's helpful to give yourself a historical context that defines your search. This could be a range of dates. It could also be determined by geographical sites of origin. You might also be interested in a specific type of source. Some examples include:
Databases and digital repositories can offer a multitude of paths to potential primary source material for your research. In addition, you might want to try searches in the CLIO: Catalog or Articles, limited by date of publication.