Where to buy the paperback?
Bookstores in most major countries should have the book in their database and will be able to order the book from their suppliers. Additionally, thanks to Wiley Distribution Services, the book can be shipped to nearly every country in the world! The most generic way to get the book is directly through the publisher Amsterdam University Press.
Alternatively, anyone in the world can enjoy the free online versions. You can download the e-book directly from the publisher, or use www.introtoquantum.org to read it as a web page (also works great on phones!).
Below are some of the larger bookstores that I know about. The list is far from exhaustive!