Primary means: "In the context of academic research or writing: designating source material contemporary with the period or thing studied; designating an original document, source, or text rather than one of criticism, discussion, or summary" ("primary, adj. and n." OED Online, Oxford University Press, September 2020, oed.com/view/Entry/151280.).
Because of efforts to digitize primary source materials, an Internet search can often yield relevant results. However, exercise caution and discernment when evaluating these results. Besides consulting with your professor, a tool to help you determine if an online source is acceptable is available here.