Some of the large databases of quotations that are readily available online are filled with misinformation. Typically no citations are provided to substantiate attributions.
Here is a small set of useful resources that will guide you in the right direction:
The Yale Book of Quotations (2006) by Fred R. Shapiro, Yale University Press, New Haven.
The Dictionary of Modern Proverbs (2012). Compiled by Charles Clay Doyle, Wolfgang Mieder, and Fred R. Shapiro, Yale University Press, New Haven.
The TwainQuotes website of Barbara Schmidt provides high quality information about quotations attributed to Mark Twain.
The Quote Verifier (2006) by Ralph Keyes, St Martin’s Griffin, New York.
Brewer’s Famous Quotations (2006) and Cassell’s Humorous Quotations (2001) are valuable reference works that were edited by Nigel Rees who is a top quotation expert and host of a long-running BBC radio program called “Quote… Unquote”. Rees is the author of many works on quotations, anecdotes, catch phrases, epitaphs and other topics. He also publishes an informative newsletter called “Quote… Unquote”.
Wikiquote, created by volunteer editors using the same crowdsource strategy as Wikipedia: This is a valuable resource worth consulting, but it does occasionally contain errors. Try to double-check the information. If the quote you are looking for is not on the “Main” page then examine the “Talk” page to see if the quote has been mentioned in this discussion section. This may give you some insight into the current status of the quotation.
Oxford Reference from Oxford University Press: This subscription service provides access to The Oxford Dictionary of Quotations, The Oxford Dictionary of Scientific Quotations, The Oxford Dictionary of Modern Quotations, and other references. Available via some library gateways.
Sometimes original research is required to trace a quotation. A new generation of valuable massive text databases has been constructed in this century. For example, the Google Books database is enormously useful and access is free. (Access may vary by country.) Items that are still under copyright are usually displayed as snippets and that makes research more complicated.
The Google News Archive is another very helpful database that is provided free. Other worthwhile databases and database companies include: GenealogyBank, Newspapers.com, NewspaperArchive, ProQuest, NewsBank, NewsVault and many others. Access is often restricted and various fees may be charged.