UC Copyright: Copyright Permission Resources
Although compiled in 2003, this University of California guide still contains plenty of live links to permissions resources. The "Guides for locating copyright holders" are good additions to anyone's list of resources concerning places to look for copyright holder information.
I particularly like the link to Kenneth Crews's page:
Crews, Kenneth D. When You Cannot Get Permission: Dealing With the "Dead End" of a Copyright Quest. Copyright Management Center, Indiana University-Purdue University, Indianapolis (IUPUI). 2002.
Copyright Permission Resources