ETR, I recommended this site in your other post, but you might not have seen it: http://grammar.about.com/od/shortpassagesforanalysis/A_Scrapbook_of_Styles_Passages_for_Rhetorical_Analysis.htm
It is really really an excellent source of passages for analyzing a variety of styles and rhetorical devices, especially since you can scroll through and choose a passage based on what style or device you're discussing in class. Most are non-fiction, but some are fiction and memoir. I used Pat Conroy's "Confessions of an Ex-Catholic" last week.
When I want my kids to read argumentative essays I usually go to the New York Times. They have a section called Room For Debate that has short editorial essays on a variety of topics. Generally have 5-6 essays for each topic and the writers will take a varity of stances. These are GREAT jumping off points for getting them to write argumentative essays. They're short and to the point -- precicely the length of the essay the students themselves will be asked to write on the exam -- well written, and interesting. We did the one about unpaid internships last Monday. http://www.nytimes.com/roomfordebate
Be careful about sticking just to modern essays though. The AP Language and Comp exam will cover lots of ground. We've seen Jonathan Swift, Walt Whitman, George Orwell, Louisa May Alcott Albert Einstein appear on previous exams. Any prose, from any time period, is fair game on the multiple choice and essay portion of the exam. The students might not enjoy the older stuff, but they have to get familiar with less modern writers because they will almost certainly come up on the exam.