You can set up bookwants with amazon.com whereby any new listings containing keywords which you've specified will cause them to send an email with the books' details so you can get it.
Here's the link:-
ebay do the same kind of thing called 'want it now"
Sellers can search through 'want it now' listings to see if anything corresponds with their stock.
Abebooks also do something similar but you need to have got a zero hit from their searchform to be presented with the link to post a want.
One more thing with regard to ebay; you can 'add favorite searches' (this option is on the same line as the number of hits given for the search result) and get the option of having details of new listings emailed to you that comply with your search terms.
(by ironsinthefire for everyone)