When you’re a comics reader, you learn to make the Internet your friend. Before the Internet came along, it was hard to find any comic book published more than ten years prior. Now, it’s a little bit easier.
For "graphic novels" (i.e., comics in book form) I usually visit Amazon first. Thousands of books that are no longer in print can still be found for sale (in either used or new condition) by Amazon's many resellers. Alibris is another option, as is AbeBooks, and even eBay (though you'll probably find what you're looking for much cheaper on the three book-centric sites--unless the book is particularly rare).
Comic book specialty retailers tend to focus more on selling the 32-page, superhero-style comics, but they may also be of some help when looking for out-of-print books. I prefer Mile High Comics and MyComicShop.com, though Bud's Art Books can also sometimes have some interesting stuff.
Fortunately, many public libraries have started filling out their comic book sections, and may also have what you're looking for.