I agree that buying the CB book is a waste of money. You can download the pdf files for the official practice tests from the College Board website and then print them out.
Online practice can be good enough for some people, but for some people it will be better to do also exercises on paper. For example, some people will benefit from underlining reading passages, crossing off answer choices, drawing on top of math problems, etc.
Khan Academy does not have an option for printing out practice material. However, I am sure there are printable online collections available in the usual places.