Keep a list of sites that you may have never heard of.
http://www.refseek.com - a search engine for academic resources. More than a billion sources: encyclopedias, monographs, magazines.
http://www.worldcat.org - a search for the contents of 20 thousand world libraries. You will find out where the nearest rare book you need is.
https://link.springer.com - access to more than 10 million scientific documents: books, articles, research records.
http://www.bioline.org.br is a library of scientific journals on bioscience published in developing countries.
http://repec.org - volunteers from 102 countries have collected almost 4 million publications on economics and related sciences.
http://www.science.gov is an American state search engine for 2200+ scientific sites. More than 200 million articles are indexed.
http://www.pdfdrive.com is the largest site for free download of books in PDF format. More than 225 million names are declared.
http://www.base-search.net is one of the powerful search sites for academic research texts. More than 100 million scientific documents, 70% of them are free
