It depends on your websites content. Stay away from ebay! That's the only real advice I can give you. Ebay is full of "opt-in" lists and half of them are junk mail/spam and unsolicited. I should know, I used to sell them. Google it, you will probably find some decent companies selling 50-100 e-mails but they won't come cheap! If you come across "10 million e-mails for $10" then you know it's too good to be true!