Hi Tracy,
It depends who you are after but I use fanmail.biz for which is free. StarTiger is also good for actors etc. There's is a fee for this site but I don't think it's expensive.
If you're willing to put the time in, you can look on the websites of agents. It's also a good way to confirm you have the correct address too.
Good luck with your requests and hopefully we'll see a few on here in the future.
Suzanne.