To find primary sources in science, look for experimental or clinical results with a significant number of subjects.

Primary sources are the leading edge of research--the first discussion of what is being found out. They may use other publications as support, but they will contribute new knowledge to the field.

In general, a primary source is the first report of research, published as a pre-print, a journal article, a research report or conference proceeding. They normally include methodology, data and results, and discussion.

Primary sources in sciences will have experimental results. A meta-analysis or a literature review, though often useful, is not normally a primary source in the sciences, because each one is based on analysis of other works. The same is true of most books, though not all.

Look under your subject for the databases for your field and search in those. Others, such as Web of Science or JSTOR, cross disciplines and may also be fruitful.