Primary sources are those where a researcher can find original data.
The major source of primary information are preprint archives arXiv.org
and journals, the majority of which are peer-reviewed.
Some journals cover only specific areas while other cover a broad array of topics.
The two most important publishers in the U.S. of physics journals are:
American Institute of Physics i.e., Journal of Applied Physics
American Physical Society i.e, Physical Reviews and Physical Review Letters
Other sources of primary literature are technical reports, patents and dissertations.