Thank you

see jroth has given some good information wish you luck

If your site is already generating a good amount of traffic, then there are sites like buyads.com, adengage.com and sitescout.com to name a few.

If you're not getting enough traffic to qualify for these types of ad networks, then I would suggest using Google Adsense to let their 3rd parties advertise in your adspace.

Hope this helps.