Any of them that sell DRM-free ebooks would work. Of course, they have a much smaller selection than Amazon and you likely won't find the book you want. So it

Any of them that sell DRM-free ebooks would work. Of course, they have a much smaller selection than Amazon and you likely won't find the book you want. So it's not really a proper alternative. You either accept their TOS or you don't get to read the book you want digitally.