i would suggest looking in your local good foods/whole foods store. maybe an asian market. or get it from a sushi resturant. i think my fish prefer raw untoasted nori if you can get it. it has more of a red color. you can also find it sold on the internet.
nori is a type of seaweed. it is sold as a health food, and it comes in sheets used for wrapping sushi.
a lot of people feed it to their herbivorous and omnivorous fish. it has more calories, protein, and vitamin content than lettuce or other vegatables. imo its a good part of a balanced diet for your fish.