Nice initiative, please consider adopting IAB standard ad sizes like 300x250, 728x90, 468x60, etc so you can access to more remnant inventory.<p>Right now many ad companies use Greenpeace or Red Cross for serving <i>something</i> when they run out of ads. It shouldn't be hard to get some impressions from them, but the standard ad sizes will be a common requirement.<p>Good luck!