There is no place to download the .js files.  The .js files are stored at the thottbot website.  This is handy in that it saves your bandwidth and you do not have to install anything.  just do what the directions say and insert:

<script src="http://i.thottbot.com/power.js"></script>

in your HTML.  After that any item linked to the thottbot item will exhibit the thottbot characteristics.  For example the following code:

<a href="http://thottbot.com/i30987">Lightbringer Faceguard</a>

will do what you need.