woocommerce/templates/order/order-again.php

<?php
/**
 * Order again button
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/single-product.php
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you (the theme developer)
 * will need to copy the new files to your theme to maintain compatibility. We try to do this
 * as little as possible, but it does happen. When this occurs the version of the template file will
 * be bumped and the readme will list any important changes.
 *
 * @see 	    http://docs.woothemes.com/document/template-structure/
 * @author  WooThemes
 * @package WooCommerce/Templates
 * @version 2.3.0
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly
}
?>

<p class="order-again">
	<a href="<?php echo esc_url( wp_nonce_url( add_query_arg( 'order_again', $order->id ) , 'woocommerce-order_again' ) ); ?>" class="button"><?php _e( 'Order Again', 'woocommerce' ); ?></a>
</p>