The instructor's edition of a textbook is usually available on the publisher's website. Used copies of instructor edition textbooks are sometimes available for resale on sites like Amazon or eBay, but many instructor edition textbooks are not legally allowed to be resold. If resale is forbidden, the book will usually be labeled with a message indicating as much.
An individual usually has to have instructor credentials, such as an institution's name or education-based email address, to be able to order a new instructor edition of a textbook from a publisher. These books are sometimes called "desk copies," and the publisher often sends them free of charge to an individual who has placed a class-size order of textbooks. Alternately, an instructor can use her credentials to request a desk copy before ordering an entire set of textbooks.
Instructor editions may also be available as e-books to be downloaded along with a class-size order or purchased separately. Occasionally, previews of instructor edition textbooks are available as PDF files on publishers' websites as well. There is also a range of digital resources available through public and academic libraries for finding various editions of textbooks, including student and instructor versions. Many of these resources are open-access for teachers and students, so both groups have access to the same information.