The standard book of popular quotations is Bartlett's Familiar Quotations, which has seen at least 17 editions so far. Bartleby.com posts electronic versions of the ninth edition from 1901 and the tenth edition from 1919. Bartelby.com also post a searchable edition of Columbia Quotations, Simpsons Quotations and the Oxford Shakespeare -- great for finding quotes by the Bard.
Another notable title is the Yale Book of Quotations. There are many other quote books, and some are available in almost every book store (see "Book Stores").
You could also try less conventional methods, such as searching the many classic works posted by The Gutenberg Project. A general search engine might do the trick, but double check to make sure the result is accurate.