You can usually download books that went out of copyright on the kobo website and other E-reader official websites. I'm not sure if they are compatible or not
Where can I download free ebooks for my kindle?
You can go to your local library and get a library card, ask them for their digital website. I get the majority of my books from there!
Try Freebookhaven. They have all the latest book titles available. If they don’t have the book you are looking for, you can post a book request and they upload it very fast :)