One way to search for newspapers is to enter the term newspapers in the main Search box on the homepage and then select Databases-- this will provide a list of USC databases that index newspapers, including some with full text content.
Here are several databases that index historical newspapers:
America's Historical Newspapers Includes over 900 newspapers published between 1690-1998
19th Century Masterfile The largest resource for pre-1920 studies, with over 6 million citations online. Search relevant printed index to material from 1800-1899, including for newspapers and periodicals
C19: The Nineteenth Century Index Draws on indexes such as the Nineteenth Century Short Title Catalogue, The Wellesley Index, Poole's Index and Periodicals Index Online to create integrated bibliographic coverage of over 1.4 million books and official publications, 64,891 archival collections and 15.6 million articles from over 2,500 journals, magazines and newspapers
California Newspaper Project An indispensable finding aid, the California Newspaper Project is an 18-year effort to identify, describe and preserve California newspapers. Close to 9,000 California newspapers were inventoried in over 14,000 repositories throughout the state, 1.5 million pages of California newspapers were preserved and made available on microfilm, and 100,000 rolls of negative microfilm rolls are being processed for permanent storage at the UC Regional Library Storage Facilities
Chronicling America: America's Historic Newspapers This Library of Congress site allows you to search and view newspaper pages from 1880-1922 and find information about American newspapers published between 1690-present
African-American Newspapers and Periodicals: A National Bibliography (1998) Covers publications from 1827 to 1990s
Antislavery Newspapers and Periodicals, 5 vols.,
Newspaper Indexes: A Location and Subject Guide, 3 vols. - Lists newspapers by county and state, with date ranges and depository designation
Newspapers: A Reference Guide, - This finding aid has information on newspaper indexes