Sonic Impact i-Fusion 2 Portable Speakers for iPod
These are built into a case that zips up, and easy to carry.
Runs from Battery & Mains Power
Unfortuantly you will suffer on sound more for the price, with battery powered once.
For around the same price If it doesn't have to be battery powered, and just mains, this has a nice sound, and is reasonably small too, then look at
JBL On Stage II iPod speakers
Available in both black & white
Both around £100
Hope this helps.
Failing that, have a look at
http://www.apple.com/uk/thestore
click on iPods and then speakers, several on there.