Sometimes conference proceedings are published with an ISSN and work every much like other serials.
But sometimes these have an ISBN like a book. We know it's tough.
Publishers are inconsistent in how they publish proceedings. Sometimes the proceeding is treated like an individual book. In other cases a conference that occurs every year or so may be treated like a journal, so that there is only one library catalog record for all the years of that conference's proceedings. Sometimes the papers from a conference are not published and may only be available from the authors. All of these factors combine to make conference papers and proceedings difficult to find. The most important thing to remember is that if you have any difficulty finding a conference proceeding or paper, please ask a librarian for help.