Ok, considering you have enough relevant traffic, you can locate them through many ways. Get your list as suggested by awiekupo. Once you know your target, qualify them according to your assumption. For those less significant one, just e-mail or fax your brochures.

Call the big companies, find out who's in-charge of marketing and make an appointment. Prepare your Sales Kit for the meeting.

Your sales kit should contain:

Info about your company/website
-mission, vision blah blah blah
-provide some background...

Data about your web traffic(google analytic stuffs)
-tell them where they come from
-what do they want, what they're looking for (run polls and questionnaire in your site now, you gonna need it)

Your price-kit
-Contains your ads space pricing according to location, size and duration
-Info on any special discounts or package

Bring a laptop and do a powerpoint presentation. Let them think first and call back about a week later to check. That's the standard I think in advertising industry..any other suggestions are most welcomed..