Maybe you can ask a friend to loan you the book? That's what I did when I didn't have my collection yet. One of your friends is bound to have a copy, I daresay it's pretty popular.
Maybe you can try Ebay if you can, they sell for cheaper online. It's okay that you saw the movie before reading the books. You can always go back to the books, read it yourself, and then fill in any missing details the movie didn't cover. It's still fun that way. Unfortunately, the movies tend to gloss over many of the finer details of the books. This is why I prefer reading the books over watching the movie. I really hope that they do the 7th book justice. I don't care if I have to sit there for 10 hours watching both movies, everything has to be included!