Javascript example
HTML
<div id="main">
<input type="button" onclick="addSelectBox()" name="clickme" value="Add Select Box" />
</div>
JavaScript
function addSelectBox(){
var parentDiv = document.getElementById ("main");
var selectElement = '<select>'
for (var i=0;i < 6000;i++)
{
selectElement+= '<option value="'+i+'">---- ' + i+'</option>'
}
parentDiv.innerHTML+=selectElement;
}