Google Scholar Many conference proceedings are open access, including many important ones not in the databases.
British Library Access to records for approximately 4.9 million unrestricted reports available for public use, including: British research and development reports, US agencies, such as DOD, DOE, ERIC, NASA, NTIS , International agencies, such as INIS, ESA, FAO, EU. Also contains the records of 400,000 conference proceedings held in stock, with 16,000 new records added each year.
Web of Conferences Web of Conferences is a scientific conference portal that collects both open access & subscription based conference proceedings series and journals published by EDP Sciences. Heavy emphasis on STEM, but selected Social Science conferences are represented.
IOPscience is an online service for journal content published by IOP Publishing. Sample search for 'leadership' https://iopscience.iop.org/nsearch?terms=leadership . https://www.plos.org/ The Public Library of Science provides free access to the full text of all their highly rated journals. Directory of Open Access Journals https://doaj.org/ Sample search- 'proceedings leadership'