If, after you've assembled your list of sources, you feel your coverage of the subject is a little "lightweight," 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 cite the same item, and it's relevant to your topic, see if that work is available and would meet your criteria.
If your list of sources seem a little scattered and unrelated to each other, you might want to think about scoping down your topic to something more specific.
Ask for library assistance to get more ideas.