The PERiodical Source Index (PERSI) is the largest index to genealogy and local history periodicals in the world. It was originally created as print indexes by the Allen County Public Library Foundation in 1986, but is now online at findmypast.com.
Findmypast.com describes it as:
PERSI provides a simple way to access articles, photos, and other material you might not find using traditional search methods. This can help to build the historical context around your personal research, and the world your ancestors lived in.
Today PERSI is available to subscribers at findmypast.com.
NOTE: Once you find an interesting article in PERSI, you can use WorldCat to discover the closest library having a copy of the publication containing that article.