You can get lots of sound effects from Sound Effects Download sites related to any season not only for Halloween. For now, try to look for the best site that offers a free download of Halloween sounds and others soundtrack you ever wanted. Try to visit A1 Free Sound Effects website or Sound Dogs, you can have it for free.
If you want to have it just for this moment, you don't have buy one because you can only used that for now and probably by the next Halloween season. Try to be practical to have your sound for free.
