Core JS does not yet have a released version, but it is available from Subversion. The JS files are also provided in Echo3 downloads.
The latest Core JS source code is available from the Subversion repositories. Please note that builds checked out from Subversion will likely be less stable than released versions.
Core JS Framework: https://svn.nextapp.com/svn/corejs/trunk
More information about using Subversion is available from the Subversion home page at http://subversion.tigris.org/.