Where do you buy (or get) e-books from besides Amazon?

So I recently got an e-reader and have started collecting e-books for it, but a lot of books seem to only be available through Amazon/Kindle. I don't want anything to do with that company. Where do I even start looking? I have my local library apps and have scoured Project Gutenberg and some similar sites. While this is great for classics and older stuff, I want newer books too, specifically science fiction and fantasy. Have looked at author's websites but they typically link to Amazon or physical copies.