Use the get_user_by
and get_term_by
functions:
// user
$user = get_user_by( 'login', 'someuser' );
if( $user ){
echo get_author_posts_url( $user->ID );
}
// tag
$term = get_term_by( 'name', 'Sometag', 'post_tag' );
if( $term ){
echo get_term_link( $term );
}