Start at the Penn Libraries' homepage.
Using Archives: a Guide to Effective Research, prepared by the Society of American Archivists:
ArchiveGrid is an index consisting of over 5 million records describing archival materials in over 1,000 institutions (archives, libraries, museums, and historical societies) including the American Philosophical Society and Smithsonian collections. Be aware: not all institutions with archives participate in ArchiveGrid, for example, the United Nations.
University of Pennsylvania Finding Aids allows you to find and view finding aids for the Penn Museum Archives, the Rare Book & Manuscript Library in Van Pelt, Archives at the Library of the Katz Center for Advanced Judaic Studies, the Barbara Bates Center for the Study of the History of Nursing, Annenberg Library Archives, the Biddle Law Library, and the University Archives and Records Center. Not included at this site are the Architectural Archives (https://www.design.upenn.edu/architectural-archives/) on the lower level of the Fisher Fine Arts building and the Pennsylvania Hospital Archives (http://www.uphs.upenn.edu/paharc/collections/archive.html).
Philadelphia Area Archival Research Portal (PAARP), formerly the PACSCL Finding Aids Site, provides access to descriptions of more than 5,000 collections from over 200 regional institutions documenting the region's vital role in our collective history,
BEST PRACTICES FOR RESEARCHERS