It's easy to Step down a 12V or 9V battery pack to 5V with a 7805 Regulator, but those oldfasioned things are not very efficient. For a simple task they might even destroy more energy then the actual circuit behind it.

Now, the 5V/5A UBEC from HobbyKing is actually a nice solution for small circuits.

It houses actually two 2.5A regulators in one package. and that for less then $7.

And if you need another voltage then there is an easy hack to achieve this.