You can get people to sign up for something... Or you can buy them off of people that sell to 3rd parties... Other than those, im not sure though
have you tried going to random fan sites? sometimes they have a guestbook that contains alot of e-mail adresses.
There are more options 1) Use those chain-mails you always get. Most of the n00bs send them to their complete friendlist with all of them included in the address bar 2) Make something on the internet people should sign up for (forum, service... whatever) 3) Have a spiderer on internet that checks for example guestbooks, forums and that kind of stuff. You can filter email adresses by the @ in the middle :') 4) Buy one. This lists are for sale. Thats where those nasty spammers get all those email addresses to spam. They're quite expensive though.
if you have the patience you could always blog hop and steal emails from tags if your target group are teens..