Usually, when I’m looking for the ebook, I check 5 sources:

Library Genesis - https://libgen.is/

Z-Library - http://loginzlib2vrak5zzpcocc3ouizykn6k5qecgj2tzlnab5wcbqhembyd.onion/

VK - https://vk.com/docs

RuTracker - http://rutracker.org/forum/index.php

Flibusta - http://flibusta.is/a

If the book is in English and it is not in these 5 sources, check Internet Archive. They have a lot of digitized books because they cooperate with American libraries:

https://archive.org/

The book can only be available for rental for an hour. This script helps here:

Internet Archive

If the book is not in all of these sources, last chance is googling it. If there is no book in Google, then most likely the book was never digitized. You can digitize it yourself. See Book digitization.