I used part # 273-054A, a mini-buzzer off the shelf at Radio Shack at $2.79. This one is 3-7 volts, but they also come in 12 volts or greater. Just wired it between two of the flasher poles. Even tucked up under the dash it is plenty loud to serve as a reminder the turn signal is on. If it's too loud, put a piece of tape over it.
