I stopped using Amazon a while back, but it was where I got all my books for a long time. I do thriftbooks mostly now, and try to buy directly from publishers when it’s a newer book, but I’m always interested in finding new spots to cop some sweet books.
FYI Kobo is the North American brand name for Rakuten made eReaders. In Europe they're sold as Tolino, with a slightly different OS/GUI as well.