Get list of all custom tanonomy id

You can get it like this

$terms = get_terms("my_taxonomy");
$count = count($terms);
if ( $count > 0 )
{
    echo "<ul>";
    foreach ( $terms as $term ) 
    {
        echo "<li>" . $term->name . "</li>";
    }
    echo "</ul>";
 }

Otherwise you can also use like this

$term_slug = get_query_var( 'term' );
$taxonomyName = get_query_var( 'taxonomy' );
$current_term = get_term_by( 'slug', $term_slug, $taxonomyName );
$args_list = array(
   'taxonomy' => 'item', // or use $taxonomyName for all taxonomies
   'show_count' => true,
   'hierarchical' => true,
   'child_of' => $current_term->term_id,
   'hide_empty' => '0',
   'title_li' => '',
   'echo' => '0',
);
echo wp_list_categories($args_list);