Front End Dropdown Showing All Custom Post Type ‘Agent’

try this:

//first get all agents
$agents = new WP_Query(array('post_type' => 'agent', 'posts_per_page' => -1, 'order' => 'ASC', 'orderby' => 'title'));
//then just print out the dropdown:
if ($agents->have_posts()){
    echo '<select id="agent">';
    while ($agents->have_posts()){
        $agents-the_post();
        echo '<option value="'.get_permalink($post->ID).'">'.get_the_title($post->ID).'</option>';
    }
    ?>
    </select>
    <script type="text/javascript">
    <!--
        var dropdown = document.getElementById("agent");
        function onCatChange() {
            if ( dropdown.options[dropdown.selectedIndex].value > 0 ) {
                location.href = dropdown.options[dropdown.selectedIndex].value;
            }
        }
        dropdown.onchange = onCatChange;
    -->
    </script>
    <?php
}