DVDdoug:
Police tasers operate off of small batteries and once in awhile someone dies.
Generally as a result of the load of drugs in their system, or other things used besides the taser. 
wvmarle:
Electric fences that are used to keep cattle out (or in) tend to work at voltages like 10-20kV, give you a good shock, but are basically considered safe to humans due to the very low current they can produce. One of such devices may be a suitable starting point. They're kept safe by limiting both the maximum current and the duration of the shock.
And when you say "duration", the frequency of the impulses is important. They allow sufficient time between impulses for you to let go and move away.
wvmarle:
Other devices that are designed to deliver safe electric stimulation (as safe as it can ever be, of course) can be found in the sex shops. Such electrical sex toys are generally advised to be used below the mid riff only. Nipple to nipple while obviously attractive is a risky idea, as the heart is right in between.
Can't say I would have naturally thought of that as a source of such equipment ... 
wvmarle:
One thing I would NEVER do is attempt to build such a device myself. A minor mistake and your party gag turns deadly. You would also need very good testing equipment to make sure the currents and shock duration are what you expect.
Or more accurately, advise a beginner to do so.
Hey, we had great fun back at school. 