Start with the people you know to see if anyone is "Six Degrees of Kevin Bacon" to them and can get you the contact information. If not...
If they are in a union you can contact the union. You may be sent a directory, given information over the phone, or be allowed to send a letter which they will forward.
If they have a production company, you can get that information from IMDBpro, the Hollywood Creative Directory, and in some cases, Google.
Last resort would be to go through their agent if they have one. Before investing the time of researching them, call the agency to confirm that they DO forward their client's mail.