You can also get royalty-free effects from:
FreeSound.org & FlashKit.com
Sometimes it's just easier to do your own, though.
Most record stores carry a selection of special sound FX (all kinds, and usually found where you'd get your Halloween Spooky Sound CDs), but they also usually have copyright notices slathered all over them so not a good choice if you really want to stay above-board and avoid possible future issues.