This buffer preamp was designed for piezo pickups, but it can also work for magnetic pickups to isolate the pickup from cable capacitance. Most any FET will work, but the 2N3819 is easily available from Radio Shack, as are the other components, The circuit will run for a year from a 9-volt battery if you turn it off when not in use.

Note that if the circuit is built right, the FET's output will bias to about half the supply voltage. In other words, the point where the 56k resistor and the 0.1uf cap meet should read about 4 to 5 volts. The circuit board should be mounted as close as possible to the pickup, optimally less than a foot away, so mount it inside of the guitar. Shield the entire circuit board to reduce hum pickup, and keep all leads as short as possible. The 10uf cap increases the gain; if the output is too high or you get distortion, you can remove it or add some resistance in series. Have fun!