Fanmail can be sent to Haley care of American Idol, at the following address:
c/o American Idol
7800 Beverly Blvd, Suite 251
Los Angeles, CA 90036
Additionally, someone has already made Haley a Facebook fan page. Her mom has commented there several times, so if you have questions, it’s a good place to start!
For Brazilian fans, someone’s set up an Orkut community page.