It really depends what you're going to hook up to the EZ-B. Have a read of this section if my power supply tutorial. There's also a detailed battery section as well. It should hopefully tell you everything you need to know. Bottom line is, if your connecting servos to the EZ-B, you will need a supply with a high amp output depending on what you have connected, and a voltage output between 4.5v and 16v.

Here's my power supply tutorial.

synthiam.com/Tutorials/UserTutorials/170/1

For portable, I would use a lipo battery. For wired, I would look at the available supply's in my thread. My thread has most of the info you need.