Let's say, for instance, that you were looking for the 9th Edition of Campbell's Operative Orthopaedics that was published in 1998. (The current one, the 10th edition, was published in 2002.) Your med-school library doesn't have it. How can you get it? Did you know that many libraries at educational and other institutions belong to a service called OCLC. (OCLC is operated by Online Computer Library Center, Inc.)
Libraries which belong to OCLC, and there are over 43,000 of them in 86 countries and territories, enter their purchases when they buy new books, and many of them have entered much of their prior holdings from years back. A librarian in a library that subscribes to OCLC can search its database and determine whether there are any libraries anywhere in the world that have listed the set.
Librarians can make what they call an Interlibrary Loan from almost 7,000 of these libraries and have the holding library send the set to the library you are using. The normal charge for this service is nominal – probably less than $20. It might take two weeks or so to get a book, but, it's a way to obtain a volume that you can't find anywhere else.
To make obtaining the book even easier, OCLC's database shows which libraries have the particular volume. If there is a library in your area, you can simply go yourself or send someone to check out the volume or copy whatever pages are pertinent.
Another out-of-the-box solution is to use one or more of the various out-of-print search services to try to find the volume. And you can search on Google for "out-of-print" to find the various services.