Tunnelblick is free software licensed under the GNU General Public License, version 2 and may be distributed only in accordance with the terms of that license.
Tunnelblick source code is available on Tunnelblick's GitHub site. To download a .zip file containing the current source code (from the master branch), click on the "Download ZIP" button on the right side of the window. To download a different branch, first choose the branch you want in the "Branch" drop-down menu near the green "branch" icon on the left side of the window.
Instructions for building Tunnelblick from the source code are included in Building Tunnelblick from Source Code.markdown, which is included in the source code.