Assuming your product type is Product-General, then look inside:
/includes/templates/template_default/templates/tpl_product_info_display.php.
Make changes to the file according to Kuroi's instructions as follows.
Delete the line:
PHP Code:
<?php echo (($flag_show_product_info_quantity == 1) ? '<li>' . $products_quantity . TEXT_PRODUCT_QUANTITY . '</li>' : '') . "\n"; ?>
Replace with:
PHP Code:
<?php if ($flag_show_product_info_quantity == 1 && $products_quantity >= 1) echo '<li>Available</li>'; ?>
Do remember to use the overrides for your changes. You may also want to look at the following mod if you want the same functionality to apply to your product listings as well.