locate_template

function locate_template( $template_names, $load = false, $require_once = true ) {
    $located = '';
    foreach ( (array) $template_names as $template_name ) {
        if ( ! $template_name ) {
            continue;
        }
        if ( file_exists( STYLESHEETPATH . '/' . $template_name ) ) {
            $located = STYLESHEETPATH . '/' . $template_name;
            break;
        } elseif ( file_exists( TEMPLATEPATH . '/' . $template_name ) ) {
            $located = TEMPLATEPATH . '/' . $template_name;
            break;
        } elseif ( file_exists( ABSPATH . WPINC . '/theme-compat/' . $template_name ) ) {
            $located = ABSPATH . WPINC . '/theme-compat/' . $template_name;
            break;
        }
    }
 
    if ( $load && '' != $located ) {
        load_template( $located, $require_once );
    }
 
    return $located;
}