When you are attempting to verify or find a quotation, it is possible that someone has already done the research for you. See these websites for "quotation experts"—writers and editors devoted to finding the origins of familiar quotations.
O'Toole's Quote Investigator website boasts more than 4.6 million users per year and includes more than 1,300 posts about O'Toole's in-depth research into the dubious origins of popular quotations. Search the website to see whether O'Toole has found the origin of a quotation you are researching.
Popik is a contributor to the Oxford English Dictionary, the Dictionary of American Regional English, the Historical Dictionary of American Slang, the Yale Book of Quotations, and the Dictionary of Modern Proverbs. On his website, The Big Apple, Popik explores the history of quotations and everyday sayings.
Rees is the creator of the BBC Radio show "Quote... Unquote," which has been broadcast in the UK since 1976. His website includes a substantial archive of material about quotations and popular phrases and sayings.
The American dialect society is "dedicated to the study of the English language in North America, and of other languages, or dialects of other languages, influencing it or influenced by it." Their ADS-L email list is open to non-members.
The Ask Historians community on Reddit can be a great source to ask questions about the source of quotations. Unlike many communities on Reddit, the rules for Ask Historians require answers to be "in-depth, comprehensive, accurate, and based off of good quality sources." Answers that do not contribute to informative historical discussion and/or do not provide sources are removed by moderators.