Which advertising agencies to sell ads space directly?

[Automotive site]

I have this site that gets around half a million page views a month and is a quality site (at least I think so and have several thousand subscribers) and am looking for an website advertising agencies that will act as middleman to sell advertising space on my site. Searching for advertisers can be quite time consuming and I would rather spend the time creating content and doing research. I know the really big sites use such agencies to sell their ad space, but I am looking for agencies that will work with moderately trafficked site such as mine.

Does anyone know where I can find these agencies. I have tried looking for one or two but simply do not know where to start.

I know some people will suggest using adsense, yes, I do do use adsense, but I have some spaces where I would like to use for direct advertsing as opposed to adsense because adsense CTR on the site is not all that high and I feel I could make much more if I sell advertising through these agencies.

My site is in the automotive niche.

[C7Mike]

Two thoughts:

a. Put your traffic on an exchange (http://en.wikipedia.org/wiki/Ad_exchange - some good links there), or contact an ad network and see if you can be a member of their network.

b. Sign up for an affiliate program and use that ad space to promote the affiliate product.

[Digmen1]

Automotive Site
I know how you feel.
I am in the opposite position to you.
We have a product that we want to advertise (cheaply) yet Google Adwords does not seem to work for us. Facebook did, but its too dear per click.
I have tried to get on to some sites and have emailed them or their agents, but I get no replies !

Regards
Digby

[edited by: tedster at 12:25 am (utc) on July 14, 2009]

[Tom Brosnahan]

Have you considered Google Ad Manager? It doesn't find advertisers for you, but it does everything else: you set up your ad slots and placements, get the advertisers' creative, and Ad Manager places the ads.

I'm just getting started on it.