I’ve always struggled to find good free sounds for films. I don’t use a whole lot and the ones I do use are often random or abstract. I collect them from a variety of sources and they often have different qualities.
I was searching for a specific experimental noise yesterday and found the perfect one, hosted for free at Freesound.org:
Freesound aims to create a huge collaborative database of audio snippets, samples, recordings, bleeps, … released under Creative Commons licenses that allow their reuse.
With over 100,000 sounds, this is surely a resource you can’t afford to miss. I spent a couple of hours thinking up things to look for, listening to different samples and grabbing a bunch of inspiring bits and pieces!