How we get_post_meta without post id

$post->ID is what makes the meta value distinguished across all posts with the same meta key.

So if you want to shorthand the get_post_meta call for the current post you can do this:

function get_cuurent_post_meta($key){
    global $post;
    return get_post_meta($post->ID,$key,true);
}

and you can call it like this:

echo get_cuurent_post_meta('custom_tags'.$userID);

Now if its not the current post but any post you can do this:

function get_meta_value_by_key($meta_key,$limit = 1){
    global $wpdb;
    if (1 == $limit)
        return $value = $wpdb->get_var( $wpdb->prepare("SELECT meta_value FROM $wpdb->postmeta WHERE meta_key = %s LIMIT 1" , $meta_key) );
    else
        return $value = $wpdb->get_results( $wpdb->prepare("SELECT meta_value FROM $wpdb->postmeta WHERE meta_key = %s LIMIT %d" , $meta_key,$limit) );
}

to get the first meta value of this key just call it like this:

echo get_meta_value_by_key('custom_tags'.$userID);

and to get all meta values of this key use the $limit param ex:

$post_meta_array = get_meta_value_by_key('custom_tags'.$userID, 999);