If you DO get transducers for your guitars get the JJB Prestige 430c. It's designed for a classical guitar application, but requires no preamp or battery, is mounted internally and works fine for steel string. The 4 15mm piezos can be spread across the bridge plate to maximaize the sonic spectrum of your instrument. 60.00 is REALLY inexpensive. You'll have to pay a guitar tech/luthier extra for the installation, but it's well worth it.

Btw, they don't NEED a transducer, but will sound better with one than without, usually. That being said I've got a mandolin w/a K&K dual-point transducer that I run straight into the board w/no pre-amp, sometimes and it sounds fine. It's all in the eq.

Also, musiciansfriend.com has a bunch of small systems that are battery powered and rechargeable. You may find something there that will work for you. The days of unamplified busking are pretty much over, unfortunately.