I've always found that the best way to obtain a large (and targeted) email list outside of the obvious opt-in form on an established property is via "email traps". By posting something tempting (a free item, service, etc) on classified ad sites you're able to target specific demographics such as "parents" or "desperate men". Then you can craft appropriate messages to be sent to these targeted lists which will surely convert better than scraping a bunch of junk from anything or buying a shared list.
Just as an fyi, with 10 classified ad posts per day I acquire more than 10,000 unique email addresses every week, conservatively.
