Learning to use modifiers to refine your search results is a big key to finding 'difficult/lost/hidden' results. While most of us know about using "quotation marks", there are a butt-load more modifiers, and you can find a lot of them HERE.
Below this line is a series of other search functions that prove helpful when trying to find documents or other 'difficult to locate' files, articles or books etc:
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Someone posted these on Facebook, sharing here for reference and bookmarking for myself:
www.refseek.com - Academic Resource Search. More than a billion sources: encyclopedia, monographies, magazines.
www.worldcat.org - a search for the contents of 20 thousand worldwide libraries. Find out where lies the nearest rare book you need.
https://link.springer.com - access to more than 10 million scientific documents: books, articles, research protocols.
www.bioline.org.br is a library of scientific bioscience journals published in developing countries.
http://repec.org - volunteers from 102 countries have collected almost 4 million publications on economics and related science.
www.science.gov is an American state search engine on 2200+ scientific sites. More than 200 million articles are indexed.
www.pdfdrive.com is the largest website for free download of books in PDF format. Claiming over 225 million names.
www.base-search.net is one of the most powerful researches on academic studies texts. More than 100 million scientific documents, 70% of them are free
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Further listing from a contact who asnwer a question of mine on Discord:
