Those little clams can be found all along the gulf beaches within about 10" of the water line about 2" below the sand... they don't do well as they are purely filter feeders and usually our tanks don't have enough crud floating around to sustain them.
Plus even dwarf hermit crabs find them absolutely delicious & in most cases the clam can't dig down far enough to get away. + the aforementioned possible disease/pest introduction thing.