There are many different techniques for doing this. Several of them are discussed below. Your best bet is to try the pertinent methods in this posting in the order in which they are listed (well, sort of; at the very least, please try all the pertinent methods which do not involve posting queries to soc.net-people before resorting to that).
I've listed "Direct contact" near the end of this list because, for some reason, people seem to be reluctant to call people on the telephone or write them a paper-mail letter asking what their E-mail address is, as long as there is even a remote chance that it might be found without asking. This attitude is somewhat counterproductive, since in most cases, it is much easier to get someone's E-mail address by asking them than it is by following the other methods outlined below. Furthermore, even if you do manage to find an E-mail address using one of the on-line methods described below, it is not guaranteed that the person at the other end of the line checks that address regularly or even that it is the correct address.
Therefore, if you do have a telephone number that isn't too expensive to call, or if you have a paper-mail address and aren't in too much of a hurry, you can probably save yourself a lot of trouble by skipping all of the on-line methods listed below and going directly to Direct contact.