When you have run your campaign across all publishers for a while, you can begin to build a new campaign with "Whitelist" targeting.

You can add all of the websites that converted in your initial campaign to this whitelist campaign and then bid higher for them.

This will allow you to have a really targeted campaign bidding only on sites you know are converting! As more and more sources convert in your initial campaign, you can keep adding them to the whitelist to scale it up.