@Guppyguy Just about any LFS should stock it, its usually in with the saltwater foods, though its more than ideal for freshwater fish. I use San Fransico Bay "Seaweed Salad" and Ocean Nutrition "Marine Seaweed Selects". If you use Ocean nutrition, only use the Green seaweed, not the red or brown.
Dried Seaweed makes an excellent staple for many livebearers especially mollies, where their large need for greens is usually overlooked by many owners. Dried Seaweed is high in protein and fiber. There is also the frozen variety, which obviously has a much higher moisture content. Hope this helped!.