It appears to me that the Adplanner tool does not show all available placements - only relatively high volume sites.
In the past I have entered individual sites in the Adwords UI and have been able to book placements even though the site doesn't appear in the Adplanner.
Admittedly I haven't looked into the differences in a while - I thought I'd ask here before starting my research.
Do publishers who use Adsense allowing placements by default?
Thanks