Online auctions such as ebay.com and amazon.com's auction site, serve as a great source for almost any edition of any Three Invesigators book. Hardbacks or paperbacks are available, and the prices depend on the book, what condition it's in, and other people's desire to purchase the same book. I've bought and sold on auction sites and find them to be a fun way to get books. With patience, you can usually find the book you want. Also, plenty of books are listed through vendors at both sites without having to use the auction process; so it's easy to find the books and be a little more in control of the purchase price.