What do you mean by resonant frequency? Surely the piezo must work with in the audable frequency band else how does it work for guitar pick ups?
Well in context of arduino most piezo elements are those used in ultrasonic range finder applications. Those run in the 100-150khz frequency range and are well above human hearing. They can be obtained either as raw elements or included with electronics to handle the driving and receiving amplification.
Now there are piezo buzzers used as simple sound makers outputs in some arduino projects, but I don't know if they can be adapted as input sensor elements or not.
I know nothing of guitar pick ups but if you can link to a specific electrical datasheet for one, possible we could see if it could be used as an arduino sensor and what if any frequency bandwidth it might offer.