custom wp_dropdown_categories items in wordpress

You should be able to send the desired taxonomies as an argument to the function. This is untested, but should work:

$args = array(
    'taxonomy' => array(
        'galleries',
        'places'
    )
);
echo wp_dropdown_categories($args);

For your reference, the function wp_dropdown_categories is defined in “wp-includes/category-template.php” on line 301. Take a look at it as it will help you understand what you can send as arguments for the function.