I imagine you can find them almost anywhere that sells books, I've even seen them at Target. I'm sure Barnes and Noble, Borders, Amazon.com, etc... would have them.
Have you looked at second-hand bookstores in your area? I get most of mine from there. Another source is bookmooch.com (a book exchange site) and paperbackswap.com (another book exchange site). You can also get used books from Amazon and don't have to buy sets.
I suggest BORDERS bookstore. The online site is located here --> http://www.borders.com/online/store/SearchResults?contrib=Melissa+De+La+Cruz&type=1&fromHeader=3