When evaluating the quality of the information you are using, it is helpful to be able to identify whether you are using a Primary or Secondary source. In doing so, you will be able to recognize whether the author is reporting their first-hand experience(s) or relying on the views of others. It is also important to be aware of the differences, as you professor may want you to utilize more primary sources and fewer secondary ones, and you should be aware of the differences in order to make the correct choice.
Primary Source - this is a first-person account by someone who experienced or witnessed an event. The document has not been previously published or interpreted by anyone else.
Secondary Source - this is a source that is one step removed from the (primary) original source. The author of a secondary source reexamines, interprets and forms conclusions based on the information that is conveyed in the primary source.
There are many sources of complete (out of copyright, public domain) philosophy texts available online such as:
Online books page: Over 1,000,000 public domain books available full text online. Search/browse by author, title, subject. Provides several editions of Plato's Republic (which includes Plato's allegory of the cave)
ALEX catalog of electronic texts: "Public domain documents from American and English literature as well as Western philosophy". Search/browse by author, title, date. Also possible to search the content.
Project Gutenberg: About 35,000 books included. Links to over 100,000 books via Project Gutenberg Partners, Affiliates, and Resources