This page will give you a very good knowledge about how to get in touch with a celebrity though an agent at an agency. Almost every "big name" celeb has an agent at one of these agencies. Of course, there is no guarantee that your letter will ever be seen by the celeb you're writting to because some use secretaries/assistants take care of their fan mail. One noted celeb for doing this is Al Pacino.

Here are addresses and telephone numbers to the major talent agencies in Los Angeles. If you are the ambitious type like me, you can call the agencies up and see if they represent the person you are trying to reach or not. It will save you some stamps. When the phone is answered by the agency, just say, "Client information please." and you will be connected to the client information depatment and then ask something like, "Hi. Can you please tell me if you currently represent Kevin Spacey?" or whoever.  They only take three names at a time because they are always flooded with calls. Most of the time, UTA and ICM will connect you to the voice mail and you can leave a message. Anyway, here are the addresses and numbers...

Creative Artists Agency  (CAA)
9830 Wilshire Blvd.
Beverly Hills, CA 90212
(310) 288-4545

William Morris Agency  (WMA)
151 El Camino Dr.
Beverly Hills, CA 90212
(310) 859-4000

International Creative Management  (ICM)
8942 Wilshire Blvd.
Beverly Hills, CA 90211
(310) 550-4000

United Talent Agency  (UTA)
9560 Wilshire Blvd., #500
Los Angeles, CA 90212
(310) 273-6700

The Gersh Agency
232 N. Canon Dr.
Beverly Hills, CA 90210
(310) 552-1100