Hi, Chris. This process is called faux (pronounced foe) painting. The place to start is with your friendly local independent paint store. They have the base color paint, gel stains and graining tools to let you do this job. The thing that you have to provide is some practice so you understand and feel comfortable with using the techniques to allow you to do this. It is easy to do but a little harder to do right. I have seen some steel doors that were professionally done that were beautiful. You could not tell that they were not wood until you got up very close to them.
One other way to approach this is to find a local hardwood supplier or furniture manufacturer with planing and milling equipment and have a new mantle board cut to size and you may want some details molded into the edges. This will be pretty expensive but if you find the right guy on the right day, it may not be as bad as you think. A lot of this depends on what you want. You will then have to finish the mantle with stain and clear finish.