There are a number of places where you can download e-Books. Some are free. Some require a fee. You may be asked to register. Be sure to choose the type of file you can download to the Kindle (.AZW .MOBI, .PRC.TXT). Books that are in public domain may be available as a free Kindle book. Two sites you might try are Many Books and Project Gutenberg, An overlooked advantage of the Kindle is the ability to get and read all the classics and public domain books for free.
Here are some sites of available e-Books in no particular order: