I stopped in Petsmart just for a quick look and found these:
http://www.petdiscounters.com/product.php?productid=1123247&cat=995&page=1
I put them in the tank yesterday and today they are absolutely decimated… just piles of powder.
Now I feed my crabs calci sand, egg shells and actual crab legs from king crabs and none of that has been eaten so enthusiastically as these things were! I paid more at Petsmart for them though. Of course if you are going to shop for them be sure to click through http://crabstreetjournal.com so that the site gets a percentage!!