There are 2 ways you can do it:
Option 1:
Joomla loads menus every time page is loads. You can access the menus by calling the following methods.
// Get default menu - JMenu object, look at JMenu api docs
$menu = JFactory::getApplication()->getMenu();
// Get menu items - array with menu items
$items = $menu->getMenu();
// Look through the menu structure, once you understand it
// do a loop and find the link that you need.
var_dump($items);
This method is faster because you don’t need to query database. Simple operation in memory.
Option 2:
Get it from the database. Either get menu link from jos_menu based on alias or something, or get article # from jos_content by article alias, then create the link
$db = JFactory::getDBO();
// Load by menu alias
$query = "SELECT link FROM #__menu WHERE alias = 'privacy-policy'";
$db->setQuery($query);
$url = $db->loadResult();
$url = JRoute::_($url);
// Load by article alias
$query = "SELECT id FROM #__content WHERE alias = 'privacy-policy'";
$db->setQuery($query);
$articleId = (int) $db->loadResult();
$url = JRoute::_("index.php?option=com_content&view=article&id=$articleId");
