You can buy live sand dollars. I think it is difficult to keep them that way, though (alive.) They need a sand bed, as that is where they forage for food (don't know if you have ever been to the Atlantic and have searched for them; I remember as a kid using my toes to go through the sand and then dive under to bring them up.) I suppose they could be used as a sand stirrer, but they are likely to preying on and consume all the available flora and fauna in your sandbed (that they can use) and then starve to death. Plus, you aren't likely to see them (as they will be under the sand.)