Primary sources are those where a researcher can find original data.
The major source of primary information are peer-reviewed journals.
These include:

• Original articles (aka, scientific papers, research articles) - longer, full accounts of orignal results of scientific investigations.

Popular Press: Material written for the general public.

Scholarly Journal: Articles written by experts in academic or professional fields.

Peer-Reviewed: Articles are submitted to rigorous scrutiny by other subject experts prior to publication.