The mantelsdirect.com website has what appears to be the exact same mantel. Is that where you saw it?

They claim to do custom sizes.

Here is a link that might be useful: Mantels Direct

I initially saw it there, but they didn't offer one for a 42" fire box (only smaller) I then did a search and located the manufacturer (they import these, and they finish the color and resell to Mantel Direct and others. They only import this size...

A custom cabinet or millwork shop would be my guesses, or maybe even a good finish carpenter. It shouldn't be too hard to find someone.