Local Calling Cards

I understand that buying a pre-paid phone card in the U.S. may not be a good idea because it may be difficult to find a phone to accept them. How easy is it to buy local phone cards in port in order to call home? Where would I buy them? Can I use dollars to buy them? We will be on a Panama Canal cruise and our ports are Limon, Panama Canal/Colon, Grand Cayman, Cozumel and Belize.

If you want to wait til you get to the port--ask any crew member what the best deal is for a phone call.. Any one of those people know where to go for the best prices.