We find these sand dollars occasionally, sometimes at VERY low tide or after a big storm. After that one hurricane we had lots all over the beach; some still alive. You aren't allowed to bring anything alive back from the beach, so I just picked up the dead ones. (You can tell if they're dead either by the fact that their little 'underhairs' aren't moving, or they turn kind of greenish.)
I know I'm being very vague in this post; I'll get some more information soon. :B