You can search for an email address using as little as a last name. To narrow the search, you'll need a first name, state and city. You can now locate several million people in such sources. If you're looking for a business, the numbers are also in the millions. The most useful starting point which I have found for locating the most people is national telephone directories.
Several sites provide national telephone directory search.
One of the best collections of gateways to such sites is The Ultimate White Pages . This site provides submission gateways and links to other locations which do the search. Two of those other locations are Who - Where and The Swichboard . There are multiple ways to find anything or anyone on the Internet. The site includes a section on finding missing relatives.