Piezo guitar pickups have been around for a while, especially in acoustic guitars. They use the electricity generated when a piezo film is deformed by the acoustic vibrations of the guitar. This guide shows how you can improvise a piezo pickup out of a few cheap parts.

You will need
1. Piezo Buzzer Element
2. about 1 foot of shielded audio cable
3. a 1/4" audio jack
4. a small amount of medium density foam.
5. soldering iron, solder, wire strippers, hot glue gun, and hot glue

Here is the link to the project: Make your own piezo pickup.

Alternatively if you want to do it properly have a look at R. Mottola's detailed instructions on making an under saddle piezo pickup for a flat top acoustic guitar. This method uses PVDF coaxial piezo cable for the transducer and also has a schematic for the necessary preamp. You will certainly get better results using this method!

Link: Make an Acoustic Under-The-Saddle Transducer