Magazines: Articles in magazines are usually short in length and are written in a popular style. An example of a magazine that has science articles is Scientific American.
Journals: Articles in journals are usually lengthy, often include an abstract of the article at the top of the page, and are written and approved by scholars and experts in the field. They often have bibliographies and extensive footnotes (which can provide leads to other sources).
Examples of journals that have articles on science are Nature and Journal of Animal Science.
Academic Search Premier – More than 4500 magazines and scholarly journals.
Directory of Open Access Journals (DOAJ) - STEM and Biomed journals. Some for a fee but most are free.
JSTOR – Full text of more than 1000 scholarly journals; including 27 General Science titles.
Nature Online - Full text of the journal Nature
Publications of the Astronomical Society of the Pacific - Full text astronomy journal, 1998 on.
Science Online - Science magazine, 1880 on.