Try these two sites to locate must-have books no longer available through standard means:
www.bookfinder.com (lists all copies in order of price, citing condition of book and cites feedback rating for each seller and where book is shipped from)
Understanding where book is shipped from is especially important if you are in a hurry to receive the book.
www.used.addall.com (used heavily by book dealers)
www.ebay.com (out of print books can appear, but make sure you are purchasing the version of the book you want)
SavvyCollectors require a substantive library to help understand more about what they are collecting.
Corinne Cain of www.SavvyCollector.com