Buy one that handles epub, Speedlever, and d/l Kindle for <your OS> and Calibre. Then you can buy epub from anywhere, or if you can only get it in mobi format from Amazon, you can rip off the drm and use Calibre to convert it to epub (if your machine doesn't also handle mobi, which few seem to).
Just don't ask how to rip the drm off in the fora. That's against the rules.