I’m pretty sure there are ways around the DRM (though that’s probably not exactly legal).

Even without purchasing from somewhere like Amazon, there are thousands (millions?) of e-books available in the .epub format and free to download. You won’t find too much contemporary stuff, but there’s a vast wealth of great literature that you can pick up at no cost. With the Calibre program (free), you can convert to a Kindle-based format.

Project Gutenberg, epubBooks, and many more are places.