On a few of my sites I've gotten emails from random people asking if they can advertise on my site. Have you ever done the opposite and emailed people and asked if they wanted to advertise on your site? For a small fee, of course. Say you have a website that is all about blue widgets. Email a few blue widget vendors and ask if they would like to be mentioned on your site. After all, if your site becomes (or is) the most popular blue widget site on the net, they'll certainly benefit from your link.