I bought something called "Music Bullet" at Walmart for $15. It sounds pretty good and gets quite loud. Good enough for the bathroom.
I've got some small Sony speakers that sound awesome. Can't recall the model # right now but they're ~$50 @ Best Buy. They can be powered by batteries or plugged in with the included wall wart.
Here they are.... Sony - Portable Speakers - Black - SRSM50/BLK - Best Buy