Finding a person's email address

Does anyone know of any sites where you can do a search to find a person's email address without having to pay for it? I'm not doing anything crazy or anything like that. I have an aquaintance, and I know they have email, but I do not know their email address and I don't want them to tell me. This person played a prank on me the other day because they think I am too smart for my own good, so I want to prove them right by getting as much free information as possible about them by this friday. Please help!