Bootstrapmade.com offers great free high quality themes for Bootstrap. You can find popular ones like Squadfree, Knight, Bizpage or Nice Admin on their site, which were already downloaded multiple hundred thousand times.
If you choose to use one of their free website theme, be aware that you have to backlink to their site. Detailed information about this you can findhere. But you also get the option to remove this backlink by optaining a license for around 19 to 39 Dollar (different plans available).