Designing Data-Intensive Applications is available from your favorite bookstore.
Here are a few suggestions. Sure, Amazon is convenient, but why not support your local bookshop? They probably won’t have the book in stock (it’s quite a niche book, after all), but they should be able to order it for you. Physical bookshops are cool, and it would be a shame if they all went out of business. <3
Sadly, O’Reilly has stopped selling DRM-free ebooks directly. The sites above are currently the only retailers known to be selling DRM-free O’Reilly ebooks.
Beware: the Google Play store sells a PDF edition of the book, but it does not have the print layout or O’Reilly design! Google appears to have converted the ePub to PDF, and the result looks terrible. Don't buy it! eBooks.com sells the properly formatted PDF.
(Many of the sites above also offer shipping to anywhere in the world, but it’s unfortunately not free.)
Note: some of the links above are affiliate links that earn me (the author) a small commission if you buy through them. If I can get paid for my hard work, it’ll encourage me to write more in the future :)