They're everywhere? Google brings back gazillions - the only real issues are what the term 'royalty free' means? It doesn't necessarily mean free as in no cost, it might mean that you pay a small fee and if you make millions with it - the ten dollars or whatever is all you pay. It could be that it's totally free for domestic use, but if it's on Youtube that could be different. If you create music and use an aggregator to get it on all the platforms, one question is always asking if you used samples, and if you did - even public domain ones cause grief.
However - that all said, you mentioned FX - so I'm guessing noises, soundscapes, percussive noises, sirens, helicopters that kind of thing? You'll find them all over the net and you then read the terms "Free for domestic use, but banned from public performance" or "not released for public performance" so you really need to be on the ball.
Images, sound and video clips are all available totally free if you look hard. Others appear to be free but you must read the small print, and above all, save a copy of the web source you got it from in case of issues. What kind of FX are you after?