You can sometimes remove rust spots on chrome by rubbing them with a crumpled piece of aluminum foil and a little water. After they're clean and dry, a coat of Johnson's paste wax over the chrome helps keep rust away. Re-wax after washing and drying the car. If the chrome is actually bubbling up, the problem is a lot deeper and you're probably best off just replacing the mirrors.
I'd suggest checking the other chrome bits on the car while you're at it (wheels, hubcaps, horn grills, turn signals).
By the way - they make convex aero mirror reproductions and they're worth it - at least on the passenger side.