Go here to search for these online journals by title:
United States History Databases:
The L.A. Times (1881-2000) offers full page and article images with searchable full text back to the first issue.
This U.S. History Websites is from EdTechTeacher's "Best of History Web Sites" and "aims to provide quick, convenient, and reliable access to the best history-oriented resources online in a wide range of categories and has been designed to benefit history teachers and their students."
This History of the United States Wikipedia article provides a good overview of the major periods in U.S. history. Use the "Further Reading" and "External Links" sections for sources you can use for your list of references (bibliography).