The dealer is going to be your best bet but I am sure it is going to be very expensive because I believe you have to replce the whole cluster.
Actually if you have to replace the cluster it may not be a good idea because your odometer is going to put a little "S" on the left side so that people will always know that you changed the cluster out.
Also depending when your car was built, it might not have the harness to do the MyColour conversion. Refer to the Mustang Source msg boards for more information, I can't remember the exact dates that can be converted.
pull the blank cover off using a small screw driver and a rag. the wires are pluged into it if you have them. cluster and switch are about $ 300 bucks its worth it because you also get the message center.
I have heard that the dealer would have to download and transfer the settings from your old dash to your new one before you can use it. This would keep your odometer correct. If that is the case, then I don't know why you would have an "S" added to the reading.
Any Ford dealer "should" be able to do this if your car is wired for it. I had mine done for $420. Check the link below for every thing you need to know about the upgrade.