MarkZ
En-Route
Bust the airspace, buzz the tower, land on a taxiway. They would be glad to give you the number.
Asking on frequency isn't a bad idea. Any of the other methods work. If you get a receptionist, ask for a fax number and fax a write-up of your request with follow up contact information. That way less information has a chance of being missed via the messenger game.
If you call a tower cab, chances are you will speak to a supervisor, controller in charge, or controller working traffic. Usually it will be someone not working live traffic.