Well so far I have gotten free ones from Amazon,
project gutenberg,
google books,
http://ebooks.adelaide.edu.au/ , and some other sites here and there.
I check these websites as well:
http://ireaderreview.com/
http://www.kindleboards.com/index.php/topic,47266.0.html
if you have a nook they offer one free book every fri
oh and if you join the facebook group they put offers there sometimes.