After you have assembled your list of sources, if you feel your coverage of the subject is still too light, and that you may be missing some important sources, take a look at the footnotes/endnotes and bibliographies/reference lists attached to the books and articles you have available. Do more than one of them refer to the same source articles or books? Would that article or book be considered important to the subject area?
Take a look at existing bibliographies, which might be a separate book, a periodical article, or posted on the Web (but only if from a credible, identified source). If several sources cite the same item, and it is relevant to your topic, see if that source is available and would meet your criteria.
If your sources seem a little scattered and unrelated to each other, you might want to think about narrowing down your topic to something even more specific.
To get more ideas, consult with a Reference Librarian at a library you are using. Take the idea back to your teacher to get further help and ideas in narrowing your topic.