You can get a new Chao by making the one you have right now run away. All you have to do is don't feed it and let it sleep (sleeping makes it more hungry). When it says, "My stomach is empty, I'm getting angry," it means it's getting close to running away. When it runs away rub the egg you have and it will hatch. Then you can buy another egg. You can get a different egg by saving and quiting. Then go back to the chao garden and you will get a different egg to buy.
Submitted by Mike the mav