Trying to find div element with id=\"result\" in returned data from .ajax() using .find(). Unfortunately, alert(res
Specify dataType: "html".
If you do not jQuery will guess the requested data type (check: http://api.jquery.com/jQuery.ajax/). My guess is that in your case response was a String rather than a DOMObject. Obviously DOM methods won't work on a String.
You could test that with console.log("type of response: " + typeof response) (or alert("type of response:" + typeof response), in case you don't run Firebug)