I work in a high school TV studio where we have a quiz segment for our televised announcements show. Not to brag, but we have an actual bell and buzzer we use for right and wrong. The bell is just a simple doorbell---don't even need electricity to run it. Just have the cover off, move the hammer, let go, and DING! The buzzer is just a regular door buzzer, hooked up to a typical doorbell transformer, which in turn is hooked to a standard-issue 120VAC plug. It can also be hooked up to a 9-volt battery, but at least for me, the transformer/wall plug setup works better. Although a fire alarm horn could work, well....don't try that at school.
Of course, if that fails, you could burn some authentic sounds to CD and play them off disc.