We will be visiting Vieques and Culebra for the first time this winter and have been trying to purchase some good maps of these islands. So far, I haven't found anything useful. Any advice would be greatly appreciated.
I do not now if you can get them in advance, however, they are available as soon as you get to either island.
Ask for them at any hotel, or restaurant.
Maybe you can search this site more in detail to see if they have one.