If you need additional sources,  you can search the Internet for more information on your topic.  Since the Internet contains material from a variety of sources--some reliable, some unreliable--you need to evaluate a potential source before using it.  The most reliable Internet sources are found on Web sites hosted by professional groups or colleges/universities.  However, be aware that even these types of sources may be biased towards a particular group's agenda.  You might be wise to consider looking at Web sites representing opposing viewpoints before you adopt any one group's information/statistics. 
For more articles, you might search www.findarticles.com for full-text articles on a variety of topics. 

For literary topics, there are some noteworthy sites (linked below) that might be helpful: 

Online Literary Criticism

Literary Resources on the Net

Poets.org

Voice of the Shuttle (Humanities resource)

William Shakespeare on the Internet

Romantic Circles Scholarly Resources (Romantic Period)

The Victorian Web

A Celebration of Women Writers