This book is fairly difficult to find. Your best bet is to regularly check on-line book dealer listings (www.bookfinder.com is a good union search engine) and the auction houses like eBay.

Right now, the cheapest listing is $65 for a library discard, which is a bit pricey. If you're patient, an ex-library copy may surface for like $20 or $30. But you'll have to move fast once a moderately priced copy appears.

There was also a Dell Yearling paperback of this title; this, too, is fairly scarce. With luck, you might find a listing for like $10.

Hope this helps. Best of luck.