I've used freesound.org very long time ago, it's a kind of general-purpose open library (musical instruments ? ambient sounds ?) so a lot of resources may be not game-y enough.
opengameart.org has a "sound" category.
If you find a sound that is mostly what you were looking for, there are many programs that let you easily cut, change pitch, and modify "envelope" (fade in, fade out...). It's quite fun!