If not having jquery this could be a solution

<script type="text/javascript">
function getElementsByClassName(classname, node)  {
    if(!node) node = document.getElementsByTagName("body")[0];
    var a = [];
    var re = new RegExp('\\b' + classname + '\\b');
    var els = node.getElementsByTagName("*");
    for(var i=0,j=els.length; i<j; i++){
        if(re.test(els[i].className)){
            a.push(els[i]);
        }
    }
    return a;
}
</script>