Many databases include them; however, to restrict your search to just papers and proceedings, try these:
Conference Proceedings Citation Index (Web of Science) Provides access to published literature from conferences, symposia, seminars, colloquia, workshops, and conventions worldwide, in the social sciences, humanities, and sciences.
PapersFirst (Worldwide Conference Papers)Over 6.9 million papers included in every congress, conference, exposition, workshop, symposium and meeting received at the British Library.
ProceedingsFirst
Online service providing access to databases and links to the World Wide Web, electronic interlibrary loan, and library collections.
