If you need to replace your firebox there are several ways for you to do this. If your fireplace is very expensive you may want to call a professional. But if you have a functional box you can replace this yourself. Before you attempt this make sure your firebox is a prefab. Due to the fact that prefab's have panels that can be replaced.
It is also recommended that you contact a professional before you start also and confirm that your box is not part of your structural fire code.
If you get the go ahead, then go ahead and get your replacement parts.