Anywhere books are sold. I strongly advocate patronizing your local independent bookstore. If you don’t have one in your area, you can order online or by phone. Of course, there are always bookseller chains and online options if that is what you prefer.
The paperback edition was released in April 2005, but with some calling around you might still be able to find stores with a hardcover copy or two still in stock.
HarperCollins has compiled a list of online booksellers.