What are you after doing?
Buy or sell...
[a] Buying...
If your buying ad's and you want to cut out the middle man you should just have to go to the site which you feel would suit your needs i.e. you dont want to go to an over 18's site when your selling baby products right....
99 times out of a 100 there will be an option to advertise... if not im sure a simple message to them would be suitable... if you would like to research there traffic to see what your asking to advertise on then you can use sites such as
www.alexa.com (search website stats or something similar)
[b] Selling ad's...
Again selling ad's is just the same process but the other way round... give people the option to get in touch with you. Let them know you are trying to sell ad's.
Other than thats there's nothing more you can do...
Alot of the time you will find people selling advertising/giving it away for free on forums... digitalpoint.com sitepoint.com and ofcourse talkfreelance.com are good places to start.
hope this helps...
Chris