Nevonexpress provides Mini Piezo Buzzer at lowest price.
Piezo buzzers are used for making beeps, tones and alarms. This one is petite but loudly! Drive it with 3-30V peak-to-peak square wave. To use, join one pin to ground (either one) and another pin into a square wave from a timer or microcontroller. For the tones, remain around 4 KHz, however, works well from 2KHz to 10KHz. For additional loudness, you can connect both pins into a microcontroller and swap that pin is high or low ('differential drive') for double the volume.