The definition of 'primary source' does differ in the Humanities, Social Sciences, and Natural and Applied Sciences. The UBC Library offer clear definitions of primary and secondary sources.
The Faculty of History at the University of Cambridge offers a virtual classroom that includes, for example, Reading Primary Sources, How to use Historical Sources, and Where do Historical Sources come from?.
As the UBC Library guide explains, a primary source is a thing that was created during the time period that you want to analyze or at a subsequent time by individuals who witnessed, participated in, and/or reflected on the events of that time. Primary sources are helpful to analyze why specific information was created and examine the relationship between the information and the event/history/culture/social norms that you want to investigate. There are four common ways to find primary sources:
1) Check appendices, notes, and bibliographies
Primary sources differ from secondary sources, which provide analysis, commentary, or criticism on the primary source. In other words, secondary sources often include various primary sources, and you can find them by checking appendices, footnotes/endnotes, and bibliographies.
2) Find primary sources using the UTL catalogue
When you search, you might include keywords/subjects, such as letter, correspondence, diaries, interviews, and pamphlets.
3) Use databases that the University of Toronto Libraries offer
4) Use images, maps, and songs, as primary sources