It shouldn't be difficult to find used copies at places like Half Price. Just about any bookstore that deals in new books will have them.
A note to people new to Narnia: Lewis's son altered the presentation order of the books. Originally, The Lion, the Witch, and the Wardrobe was the first book. Preferring the chronological order of the stories to the order in which Lewis published, Lewis's son requires that all recent editions place The Magician's Nephew first. Likewise, he moved The Horse and His Boy from the fifth book to the third spot (between Lion and Prince Caspian).
Therefore, if you purchase or borrow a mix of editions such as from used bookstores or the library, you may find the sequencing peculiar.