Ah, sod it - I hate being tight and wanting cash

Most advertising companies or online marketing companies havereally shoddy search-engine data, so finding them is a nightmare (go figure).

Look for companies such as Aurora, Doubleclick or Adtech.

I myself would look into my own Sponsorship and Advertising system - as that means you can control what adverts, where they go and how much you get etc.
Depending on what you site is built with/how good you are, you can handle such a thing yourself, (I use Xaraya and use the Articles module ).
Alternative, you can use something like adnews or what not to control your own campaigns, and that has instructions on how to include the advert display code.