Swarthmore's Friends Historical Library documents the history of the Society of Friends (Quakers) from the 17th century to the present. FHL's collections reflect the role that Quakers have played in the abolition of slavery, in prison reform, Native American rights, women's rights, humane treatment of the mentally ill, post-war relief efforts and temperance (among others).
Temple University's Urban Archives document the history of the Philadelphia area from the 19th century to the present.
For example, you could visit the Urban Archives to explore the termination of the Philadelphia Police Advisory Board in 1969. The Police Advisory Board was charged with investigating citizens' complaints regarding incidents of police brutality, wrongful arrest, or other wrongful conduct. (Collection description)
The Rare Book & Manuscript Library at the University of Pennsylvania offers a broad collection including Medieval, Renaissance, Early Modern, and Modern manuscripts.
The Library has particularly strong collections in American literature, drama, and history as well as the Edgar Fahs Smith Memorial Collection in the history of chemistry and the Henry Charles Lea Library with strengths in Church history, the Inquisition, magic, and witchcraft.
To search and browse the collections, use the UPenn Library Catalog (Franklin). You can limit your search to the Rare Book & Manuscript Library (scroll down to the section on the left labeled "Library").
Finding aids for print collections (e.g. the Atomic Age pamphlets collection, the Cushman Club collection on Philadelphia theatre history, and the WWI Printed Media and Art Collection)
A few helpful hints for working in archives, special collections, and libraries:
Consult finding aids for collection descriptions, material types (e.g. correspondence, news clippings), material dates, and a sense of how large the collection is.
Before visiting an archives or special collections library, check their hours and contact the staff to let them know you're coming. They'll probably ask you which materials you'd like to use.
Although more sources are available online than ever before, the vast majority of primary source documents (held by archives and libraries) have not yet been digitized.
Librarians and archivists are available to answer your questions, so don't hesitate to ask for help if you run into difficulty. Archivists and librarians may know about collections and sources that you haven't heard about before.
This guide from the Society of American Archivists may be helpful as you plan your research.