The most reliable source for discovering quote origins is the Oxford Dictionary of Quotations. There are also several websites dedicated to quotations that many consider reliable.
The Oxford Dictionary of Quotations is currently in its 7th edition. It contains over 20,000 quotes that have been verified. This dictionary is a reliable reference material for any scholarly paper.
Websites such as BrainyQuote.com and QuotationReference.com are dedicated to keeping reliable and updated databases of quotes by famous individuals. These websites are more up-to-date than the print dictionary, but there is also a higher chance of a quote being mistakenly associated with the wrong individual.