You can perform a search for email address in previous correspondence, public directories or using database sites like Addresses.com. Addresses.com has the largest database of email addresses in the world.
There are a variety of other resources that allow you to search for active emails, such as People Smart search. This resource compiles a report on its findings that includes the email address and contact information.
Emailfinder.com can perform a search using a reverse lookup method. Simply enter the name of a person, and in some cases the search returns contact information, social network activity, address and occupation. You can also utilize major search engines, such as Yahoo or Google, by typing the person's name into the search engine and viewing the search results. Sometimes you may need to input additional information, such as an address, nickname or hobby that is revelant to the name searched.
Individuals who are active on newsgroups such as Usenet may have left an email address on posts they've created. You can browse through the posts, mailing lists, topics and discussion groups to find the email address. For those who were in the Air Force, there is an email database that has a listing of past and present Air Force personnel.