You don't need an 'amplifier', as it's not a linear transducer, and only a squarewave feeding it.
Simplest solution would be a simple H-bridge to drive the buzzer in a bridged configuration - this gives you double the voltage, and four times the power.
I did this with a piezo buzzer for a project in Practical Electronics, although the original design used a single PIC pin to feed the piezo, I simply wired the other end of the piezo to an unused pin and rewrote the software to give a bridged output. The volume difference was substantial, and is probably all you need.