How to limit display of iframe from an external site to specific domains only

you can use an .htaccess (assuming the original content is on an Apache server) to limit the access to a specific IP.

Or, if the page is a PHP, you could limit it to a specific domain, like this:

    <?php
$continue = 0;
if(isset($_SERVER['HTTP_REFERER'])) {

    //correct domain:
    $ar=parse_url($_SERVER['HTTP_REFERER']);
    if( strpos($ar['host'], 'yourdomain.com') === false ){
    } else {
        $continue = 1;
    }

}

if($continue == 0){
    header('HTTP/1.0 403 Forbidden');
    exit('Forbidden');
}

?>