It's really easy to come by. Hard not to waste something that's so plentiful.
I find dental surgery works pretty well.
Once you hit those nerves you're set, just keep whacking them with some object (I recommend either a spoon or fork) and watch as the person screams in pain.
Just make sure that the objects you're using to hold the mouth open can withstand the force they'll put on it when you come at their face with a drill or pair of pliers.