How To Find Quotations
There are several tactics for tracking down the full reference to a quotation. It is easy enough to put a quotation into Google and find out to whom it has been attributed. It is often much harder to find a source for the original quotation, especially when the quotation has been paraphrased, poorly cited, or mis-attributed.
For more widely known quotes, our books of quotations can be invaluable. From the general and classic Bartlett's Familiar Quotations to the more specific Oxford Dictionary of Humorous Quotations, they are routinely indexed by author and by subject. These books have different areas of overlap, so be sure to browse the reference section for multiple sources if you don't find your quote in the first book you try.
The subject index is particularly useful when you know a paraphrased quote or are not sure of the author:
"I wouldn't want to be in a club that would have me as a member." -perhaps Mark Twain?
A search in Bartlett's for club as the subject leads us directly to the actual quote:
"Please accept my resignation. I don't care to belong to any club that will accept me as a member."
-Groucho Marx, The Groucho Letters (1967)
A useful free resource is: http://www.bartleby.com/quotations/