Be carefull, even if the store claims that the battery is new, it's probably a few months-> year old.
I got a battery form my clamshell a few years back (think it was in early 2002), any way the battery was over a year old, and I returned it!
As you probably know, even with todays battery technology, batteries gets old and a unused 2 year old battery can be unusable.
The clamshell batteries is one of the most expensive batteries you can get..
You probably know that the clamshells only supports 800x600 resolution?
The sure looks great, but are a bit limited for todays use..
You can get the battery here:
http://www.wegenermedia.com/ibookp.htm
http://www.smalldog.com/product/39528/ss