Firstly download Calibre to your desktop - it does work with Kindle.
http://calibre-ebook.com/
See if your library lends ebooks. If yours doesn't, a neighbouring county might and you can register with them. My library allows three loans at a time.
The only legal free books available for download are out of copyright, so are old, or unpublished where the author is happy to share.
There are many only bookshops selling ebooks, and the prices vary.