That's it really, a website of somesorts where you could find anyones address. Probably something you would need to sign up for.
in short you cant, and to prove my point to anyone that thinks I am worng please post every email address I have online apart from the ones I post on here.
Dave many people do not fill in their profile online making it near on impossible to track or find an email address.
now if it is a company your looking for that maybe different, if it is a private email address without profile then thats just not possible.