Have you tried portable PA units? These portable PAs are battery powered units and you can plug a any kind of handheld player into them.
Take a look at these sites:
http://www.dak.com/reviews/3010story.cfm
http://www.radios4you.com/pasystem-mwp2005-LG.html
http://www.musiciansfriend.com/navig...table+PA&Nty=1
The first one doesn't quite have enough bass (i own one), so I'd check out both the second and third links above.