If your ever looking for a specific sample one of the best places you could go to search would be The Freesound Project. At the time of writing this they have over 40,000 samples, all available to find via a search tool. All samples are provided free for you to use by other generous people from around the world under a creative commons license (which basically means you can use it as you like as long as you mention the person who made the sample).

I have also heard that a sample of a scream from The Freesound Project was used in the film "Children of Men" which starred Clive Owen and the user was properly credited in the credits, so if you make your own samples, why not upload them. You never know you may hear your sample in the next big movie.

I like The Freesound Project a lot and I thought it deserved a mention. I shall certainly be using it to search for sounds when I need a sound that I can’t make myself. Just remember to attribute the sample creator as great tools like this wouldn’t exist without them.