PeterH:
If you're thinking of those little PP3 9V batteries, they're convenient but a horribly expensive and inefficient way to power an Arduino - and can only supply minute amounts of current which makes them a poor choice for powering any loads. I think you would be better off looking for four AA or AAA size NiMH rechargeable batteries to power the servos/motors, and if your motors don't draw too much current you might even be able to power the Arduino from the same batteries.
As I said in another post in the Project Guidance (How to power this project - #3 by MichaelMeissner - Project Guidance - Arduino Forum), I recently got an EZOPower 5000 that provides two separate ports of 5v 1amp power.
I also have the Energizer portable battery for cell phones (Energizer Black Portable Charger for Cell Phones and More (XP1000) - Newegg.com) that is a lot smaller, and might work better for the prop, though it only has 1 USB port, and a lot less power (1000maH vs. 5000maH), so if you need to run it for an entire day at the convention, I would go for something like the EZOPower.