I’m afraid that some themes simply aren’t freely available outside of wordpress.com.
If you are specifically interested in Hemingway,
http://github.com/kneath/hemingway
The download link is at top of page. Theme author home page is also listed, (warpspire.com) follow links for information on the theme status, or view the revision history at the “history” link on the right side of that page. You may also see the commitment of updates contributed by the wordpress.com team, if that is of any further reassurance.
You might also find it in svn, http://svn.automattic.com/wpcom-themes/ but if the maintained version in the first link is more recent, or already contains the wordpress.com revisions, you might consider just downloading that instead of using svn.
