SEO is your friend. Ad networks are a tough game to throw money at. The cheaper one's will have bad quality and the really good one's probably won't fit into your budget. I say this because if you don't know what you are doing, you can spend yourself into an early death for your business.
Is your website casual games? Are they flash based or html5? I can probably help point you to some folks that might be able to help you out. I have been in the ad network game for 15 years and have helped build some of the largest ad networks in existence from very early stages. Feel free to private message me if you'd like to talk about this in more depth.