Advertisers will Find You
While there are various commercial sites where you can generate income from your site, bear in mind that there are also private ones who may want to advertise on your site. The manner of finding you is easy; they found you while searching for potential partners. While it may seem that it is a long shot, the validity of it all is the most important thing.
If you happen to get an email all of a sudden asking you if you wanted to accommodate certain requests as buying ad spots on your site or even certain posts, you will be amazed at how much people would pay you for it. One entry can make a difference and while it may seem low at first, imagine the number of advertisers who would do the same thing? In fact, the more posts you have the better since it would mean that you are offering quite a number of spots that can allow advertisers to choose from.
As far as payment for these ad spots is concerned, it would be wise to seek advance payments. For example, if they want to have a spot all to themselves for a year, payments made online like perhaps Paypal is fair enough. You can specify the size, number of words or even limitations (which is perhaps rare and dependent of the site where they will be advertised). Of course you would want ads that do not disrupt the flow of your content.
Finally, if they find you, it only means one thing. You have done a great job in marketing your site and it is safe to say that your site is optimized on the web after all.