If your source has zero impedance then the voltage applied to your bridge of rectifiers will be unchanged by the current drawn by the piezo resonator. If your source has some non-zero impedance, then the current drawn by the piezo resonator will cause a voltage drop in the source impedance, and less voltage will be available for the recitifier bridge.
The way to model the source impedance in spice is to simply put a resistor (possibly with some reactance as well) in series with the source.
What is the nature of your physical source? Is it a signal generator, or some kind of amplifier? Do you know what its internal impedance is?
To find out its internal resistance, set up your source to provide an output without a load and connect a voltmeter or oscilloscope to the output to measure it. Apply a resistive load, perhaps a variable resistor, and vary the resistance until the output of the source drops to 1/2 of the unloaded value. Then the value of the resistive load is equal to the internal resistance of your source.