The pangeros bring in crab at La Pinta estuary. You could go out there and see if anyone is bringing them in. It's a nice interesting place to visit anyway so it would not be a wasted trip. You get to see Gillespies old hacienda there. There are almost always pangeros working and they might be able to tell you who would have crab. Another long shot possibility is Moon , the big buyer of all kinds of mariscos on the street to the Malecon, right hand side, big building. They do not sell retail but You could ask !! Good luck.