In order to get admin order note, there is no office function to do that but you might want to do that to capture payment date in woocommerce. You can easily find the order note by doing the following,
looks pretty simple right? If you can't get it this way, all you need to do is to sql the database direct instead especially if you re on a multi language WordPress site.
$paymentdate = $wpdb->get_var( 'SELECT comment_date FROM `wp_comments` WHERE `comment_content` LIKE \'%payment success%\' AND `comment_post_ID` ='. $post_id);
and these should gives you the information you need to do get your payment date if you ever need it.