Drush finding 3rd party module but can't install
Please see --lock, described in drush help pm-update:

--lock

     Add a persistent lock to remove the  
     specified projects from              
     consideration during updates.  Locks 
     may be removed with the --unlock     
     parameter, or overridden by          
     specifically naming the project as a 
     parameter to pm-update or            
     pm-updatecode.  The lock does not    
     affect pm-download.  See also the    
     update-advanced project for similar  
     and improved functionality.