Hi
We returned from the Western cruise today and are frequent visitors to Key West.
The Key West Post office is within walking distance of where the ship will dock. It is on any Key West city map. They are readily available in what appear to be newpaper stands in the "whats happening in Key West" (I can't remember if that is the name)magazine. As for Grand Cayman and Cozumel, we did not mail any cards from those ports, so I am not sure of the location.
The shops that sold the cards had very friendly staff, so I am certain they will point you in the right direction, or you may even want to ask Guest Services on the Magic, they seem to know just about everything about the ports of call and I would think this would be a fairly routine question.
have a great trip