Encyclopedias & Dictionaries are a great start to get ideas. They often have a bibliography that can point you to more resources as well.
20th Century Art - The brief entry gives a very broad overview of art movements from the last century.
Below are just a few reference books on art. Note that you can also get into this topic from another direction, such as the theme or individual you want to discuss.
Fox Games. Denver Museum of Art
From your assignment:
"By November 3, you should have all your sources, including at least three print sources. " -- what does this mean to you?