Help with defer parsing script.

What do I replace with "defer.js" in the script below?
I have test page if you want to look at http://www.envisionlandscaping.com/t...k_fencing.html
Thank you

<script type="text/javascript">
function downloadJSAtOnload() {
var element = document.createElement("script");
element.src = "defer.js";
document.body.appendChild(element);
}

if (window.addEventListener)
window.addEventListener("load", downloadJSAtOnload, false);
else if (window.attachEvent)
window.attachEvent("onload", downloadJSAtOnload);
else window.onload = downloadJSAtOnload;
</script>