Be aware that they are very small (about half the size of a regular beach towel) and I have not seen them offered outside the ship at every port.....but they definately have them in bins for the taking at
Castaway Cay.
If we are on our own excursion we usally take them from the pool area on Deck 9. I also purchase a full size beach towel from the shop onboard as I collect them and I like to use a full size towel. Another good thing about your own towel is the
DCL issued towels are white and sometimes hard to spot your "area" on the beach from the water....a brightly colored towel helps you find your spot on the beach!!!
DCL does provide towels for all of their sponsored excursions and they will be handed out at your excursion meeting place onboard.