Study guides are available at most bookstores including Barnes and Noble and amazon.com. Go to the study aides section and find the CLEP book with the test you plan to take.
It is highly recommended that you take the CLEP demo to familiarize yourself with the testing process. You can find it by clicking on "preparing" on the left side of the page at www.collegeboard.com/clep
Review a textbook on the subject. You can find used ones at websites like amazon.com and half.com. You can save money by buying an older edition.