To do this, usually for the purposes of a phishing beta e-mail, it is simply a case of using information posted on the official Warcraft forums...
Simply go on a realm of your choice and look for a stickied topic, usually referred to as the 'Guild Thread' or somesuch. There will usually be absolutely tonnes of links to different websites related to guilds. Usually these websites will have or actually be forums.
From this you can either simply go about the message board clicking on usernames and a good 50% will have an e-mail on their user page.
Another way of course is to search entire forums for 'hotmail' or 'gmail' but this is much less reliable.
This can also be specific as you can see in application threads what character you would prospectively be getting. This may all seem simple but can easily be overlooked and is very handy to get a hold of accounts that are not being sold (in which case a beta e-mail is perfect)
[My first contribution, so be nice=]]
oh and for the purposes of this scam I would reccomend aestysu's beta phisher, I cannot link due to lack of posts but i reccomend.