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, limited by date of publication.
TIPS FOR SEARCHING FOR PERIODICALS USING CLIO CATALOG:
SEARCHING FOR ARTICLES USING KEYWORDS IN CLIO ARTICLES+: