Sound Bible is an excellent site for getting royalty free sound effects. I first heard about this site from Free Technology 4 Teachers, and was surprised at how easy it is to use. All you have to do is browse their extensive library of effects or type in which sound you are looking for.
Another nice feature is ability to request a sound. This is ideal if a student/teacher need to use something that is not posted. My only concern is if there are any inappropriate sounds. I did find some that would fall in the category of vulgar so I'm sure this is something that educators should be aware of.
Once you have found the effect you are looking for you can download it either in .wav or .mp3 format.
Give Sound Bible a try by clicking here.