woo_manufacturer_admin_search_field.php

<?php
function ci_admin_posts_filter( $query )
{
    global $pagenow;
    if ( is_admin() && $pagenow=='edit.php' && isset($_GET['ADMIN_SEARCH_FIELD_NAME']) && $_GET['ADMIN_SEARCH_FIELD_NAME'] != '') {
     
         $query->query_vars['meta_value'] = $_GET['ADMIN_SEARCH_FIELD_NAME'];
 
    }
}

add_filter( 'parse_query', 'ci_admin_posts_filter' );
 
//////////////////////////////////////////////////////////////////////////////////////////////////////////////


function ci_admin_posts_filter_restrict_manage_posts()
{
    
    /** Ensure this is the correct Post Type*/
    if($_GET['post_type'] !== 'product')
        return;
        
    global $wpdb;
    
    $sql = 'SELECT  post_title FROM '.$wpdb->posts.' where post_type="manufacturer" ORDER BY post_title asc';   
    $fields = $wpdb->get_results($sql, ARRAY_N);
?>
<select name="ADMIN_SEARCH_FIELD_NAME">
<option value=""><?php _e('Search By Manufacturers', 'ci'); ?></option>
<?php
    $current = isset($_GET['ADMIN_SEARCH_FIELD_NAME'])? $_GET['ADMIN_SEARCH_FIELD_NAME']:'';
    
    foreach ($fields as $field) {
        if (substr($field[0],0,1) != "_"){
        printf
            (
                '<option value="%s"%s>%s</option>',
                $field[0],
                $field[0] == $current? ' selected="selected"':'',
                $field[0]
            );
        }
    }
?>
</select> 

<?php
}

add_action( 'restrict_manage_posts', 'ci_admin_posts_filter_restrict_manage_posts' );