The easiest way to find up-to-date information on the contact
addresses and the agents/managers of film actors and casting directors
is to look up for them through IMDB Pro. IMDB Pro is a for-a-fee
database, which contains such accurate information. You can also sign
up for a free trial and get for a while the information you need
without being charged for it.
Because this is a database for a free, quoting the information in it
might violate its ToS. However, sometimes one can also find the name
of the agent and the contact address just by Googling!
If you search for [Harrison Ford] you get a fan site that contains his
contact details, including the agency that represents him:
c/o United Talent Agency
9560 Wilshire Blvd. Suite 500
Beverly Hills, CA 90212
10279 Century Woods Dr
Los Angeles, CA 90067
SOURCE: Harrison Ford Web, <http://www.harrisonfordweb.com/contact.php>.
Mel Gibson, according to the same search method, is represented by Ed
Limato, co president of ICM agency:
International Creative Management
8942 Wilshire Blvd.
Beverly Hills, CA 90211
"Mr. Limato represents Richard Gere, Mel Gibson, Denzel Washington,
Steve Martin, Billy Crystal, Claire Danes, Thomas Jane, Liam Neeson,
Paul Walker, and others. " http://www.icmtalent.com/execteam.html
Hugh Jackman is represented by Endeavor:
The Endeavor Agency
9601 Wilshire Blvd., 3rd Floor
Beverly Hills, CA 90210
My colleague Rainbow, has all the details regarding his contact details:
Hugh Jackman agent
Here are several lists of casting agencies:
Talent Agencies - from the Voiceover Guide
SAG (Screen Actors' Guild) list of Franchised Agents
Association of Talent Agencies
I hope this answers your question. Please contact me if you need any
clarification on this answer. May your big Hollywood dreams come true!