You can buy a book outside of Amazon, or want to just go to some public/institution library! 
Also some authors have an "affiliate" link on their website so you can buy book at the same price but they have a small fee (I'm thinking of Martin Fowler for example)

The ISBN can be found on any book description, for example on Amazon or on the publisher website. It was not really in the scope of the PR, after this one is merged I'm ok with adding it.