Since the PRC-100 is just a Sharp Zaurus, searching for the Shark Zaurus EA-BL08 (It says EA-BL08 right on the back of your PRC-100 also) should do the trick. Here's the first one that google returned for me.
http://www.bigupbattery.com/Sharp-EA-BL08-Zaurus-SL-C1000-Battery-p/bupb3502.htm
-Kevin
________________________________
From: pl88ks <play88keys@gmail.com>
To: disklavier@yahoogroups.com
Sent: Friday, August 23, 2013 6:56 PM
Subject: [disklavier] Mark 4 battery
Is there a current source for a PRC100 battery? Thank you!