Locating Copyright Holders: Getting Permission
Attorney Lloyd J. Jassin composed a page in 2004 about this topic which shows a good variety of ways to seek permissions. In 1998 Jassin and colleague Steven Schecter wrote: The Copyright Permission and Libel Handbook: A Step-by-Step Guide for Writers, Editors, and Publishers and this page reflects the in-depth knowledge accrued in Jassin's experience. An update would be nice, but there's much to be gleaned from this very good page.