I agree with Adii -- LaunchBit is good. They're still fairly early stage, but that gives you a leg up in terms of inventory. I worked with them for a few months and was able to achieve a relatively low cost-per-lead with them.
You could also try some affiliate marketing with companies like Commission Junction or Netline. These can get you pretty solid volume, but if you're solving for quality, I'd keep them lower on the list.