I would first try talking to your family, neighbors friends or co-workers and see if they have had this done and get a recommendation that way. We don't have one of those mailboxes, however our neighbors do. I've seen several of our neighbors do it themselves and it doesn't look too hard and doesn't take a long time. If you want to do it yourself, I'm sure Lowe's or Home Depot would have books to show you how.
Do you have a handyman or have you done any additions on to your home or major work done on your home? You might try with some of your current companies you've worked with in the past. They might not be able to do the work, however maybe they could recommend someone else to you. That might be a start.
I'm sorry I can't help you out on a personal recommendation. We are thinking of doing something similar, however we want the locking mailbox and the older style mailbox that is the heavier metal that looks like it is from England (fancy/older style of mailbox that says letters on it).
Hope this helps and good luck!