Whole Foods carries a brand called Main Coast Sea Vegetables, Dulse. Its and organic brand of red Atlantic seaweed that comes unprocessed and unwashed. It claims to come tested for heavy metals, petroleum, radioactive and microbiological contaminants, for what that is worth. It hold up very well when being fed on. My tang tends to just make a mess with sea veggies.
My local store sells it for 7 something for a two ounce bag, but that goes quite a long way.