Enter that quote into google.

If you can’t find the source you should remove the quote. An incorrect citation will probably lose you marks for sloppy research and could still be considered as plagiarism.

Don’t guess.

I agree with @Lightlyseared that the best course is to remove the quote if you can’t attribute it.

Alternatively, I would try to find it again and pick up the correct citation. Can you get to the library that the books came from? Might it be quoted elsewhere? Would your instructor possibly recognize it if you went to him or her and asked?

If what’s important about the quote is the idea it expresses, could you find the same idea in someone else’s words and use them instead? Is it a general enough idea that you could paraphrase?

If the exact words are important, or more important than the idea, then it’s probably not a cornerstone of your argument in your paper. Is it mainly ornamental? You can do without it. Googling it is worth a try, in that case, but bear in mind that a lot of things quoted on the Internet are quoted incorrectly and/or misattributed. Books are still a safer source.

I agree with @ragingloli (weird, I know.) Enter it into Google and find your source. Maybe include the word “book” in your search.

Definitely try the quote in google – and then google scholar. Use quotation marks; it should turn up the citation even if not the actual paper. If you need help, please feel free to PM me; I have a lot of experience finding obscure publications.

I’m dying to know what the quote is now!

Do away with the quote, if you can’t cite it – it’s plagiarism in any college class. Don’t take the chance on getting kicked out of school, it’s just that serious. If you can find another author in the same field use his/her quote instead. Better yet, use a minimum of quotes in any college writing, your instructor or professor is really wanting to hear it in your own words anyway! I’m in a MPA degree course right now and the less quotes used brings higher points!

Try Google first. It does work sometimes. Don’t put the whole thing in, just a few key words. Try Google Scholar too. If you can work out which book it was from (through Google or just guessing) then check Amazon and see if the books available with a “Look Inside” option. Then you may be able to find the quote and page number.

Been there done this. If all this fails, take the quote out. Universities often use text matching software. This means even if your marker skims and misses it, the matching software will probably highlight it.

Good luck! Oh and use this as a learning exercise and take better notes. The first thing you write down is the reference info, then the quote and page number. That way you are never (or very rarely) in this position. Usually happens in the middle of the night when you are madly trying to finish a paper and then you realise you don’t have all the info you need or in my case, I might want to double check the quote is correct.

google worked! Thank you, all! (that was such an obvious, but smart answer, @ragingloli)

I use it all the time to see if my students are plagiarizing. Yeah. It takes you right to the source. :) I think they may think I’m a witch. “George. You quoted this verbatim from the very obscure website “They’ll never find out you cheated.” This is your first warning. Please put in your own words.