WooCommerce: Show Contact Form When Product Is Out Of Stock

Paste those into your theme’s functions.php and edit the contact form shortcode inside do_shortcode function.

/* Show contact form instead of "Out Of Stock" message */
add_action('woocommerce_single_product_summary', 'out_of_stock_show_form', 20);
function out_of_stock_show_form() {
    global $product;
    if(!$product->is_in_stock( )) {
        echo  '<div class="oos-form">';
		echo  'This date is already sold out.<br>Contact us if you can\'t find the date you want:';
		echo  do_shortcode('[contact-form-7 404 "Not Found"]');
        echo  '</div>';
    }
}

/* This one will work on Variable Product - When a Variation is out of stock */
add_filter( 'woocommerce_available_variation', 'variation_out_of_stock_show_form', 10, 3 );
function variation_out_of_stock_show_form( $data, $product, $variation ) {
    if( ! $data['is_in_stock'] )
	{
        $data['availability_html'] = '<div class="oos-form">';
		$data['availability_html'] .= 'This date is already sold out.<br>Contact us if you can\'t find the date you want:';
		$data['availability_html'] .= do_shortcode('[contact-form-7 404 "Not Found"]');
        $data['availability_html'] .= '</div>';
	}
    return $data;
}