If you have emailed them before, you probably have their address. Go find it now.
From public records to MySpace.com to email address directories and change of address services: find people and their email addresses using dedicated search engines.
Search somebody's email address like you search for anything else on the Web (and successfully).
Find the person whose email address you are looking for in discussion groups.
If the person you are looking for is searching for herself, she will find you, and her email address will find its way to you.
Go through your collection of business cards to find email addresses of more people than you probably know.
Yes, that's obvious, but asking is still the easiest way to find an email address.
If all other means to find somebody's email address fail, you can turn to the Usenet newsgroup soc.net-people.