Where to advertise my PPC?

I have a number of blogs and technical related websites, about 20 to be exact. They get a high volume of quality traffic – the smallest of which gets about 500 unique visitors per day. I have teamed up with 3 others doing similar things and between us we have about 50 sites which get around 70,000 unique visitors per day in total(Combined Sites).
We have setup a structured approach to selling ads for the sites and we have put together some pricing and a beta program. Where can I go about advertising for customers for this program?
We are looking to initially sell $25(1000 clicks) and $50(3000 clicks) packages of ads to about 5-10 people and work closely with them to see how their results pan out, and what we can do to improve traffic amounts and quality, etc.

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