The wiki also has a list of ebook stores:
https://wiki.mobileread.com/wiki/E-book_stores
And I have had a lot of success in finding books, both ancient and modern, merely by googling. Put the author's name quotation marks, add the book title, also in quotation marks, and the word ebook.
e.g.
"Jane Austen" "Pride and Prejuduce" ebook