When I’m looking for the ebook, I check these 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 it’s not at these 5 sites, check Internet Archive. It has a lot of digitized books because they cooperate with American libraries:

https://archive.org/

The book you need can be available only for one-hour rental. This script helps:

Internet Archive script

If you still didn’t find the book, last chance, google it. If nothing again, most likely your book was never digitized. You can digitize it yourself. See How to create ebooks.