There are quite a few places on the web where you can track down out-of-print titles. Here are some places to search, with some tips about where to look for what.
These sites are simply tools for searching more than one web site (see next section) at a time. They are great time savers, and I recommend them highly as a starting point for your search.
These websites are all searchable. That means that you can input an author and/or a title, and you'll receive a list of available items.
Some of the likelier auction sites.
Newsgroups have been around much longer than the web. They are free and unmoderated - the net at its best and worst.
E-mail lists are rather like an email version of a newsgroup or bulletin board, and the same rules apply for the most part.
A few places where you can post want lists more or less permanently.