I generally buy it at a HFS. Most grocery stores usually carry Nori & Dulse, now, as well. Spirulina powder is available and is generally added to protein drinks.
It's strange, but I actually really like the taste of seaweeds. Like Retrograde (Laurel) mentioned it's wonderful blended in rice or other grain dishes of a more savory nature. I love adding to to soups, stews, stir-fries, etc. You're probably most familiar with seaweed in sushi. I even use it as a kind of ground condiment (like one might use fake cheese or a salt substitute or fake meat or fake fish).
It's really high in iron, by the way, as well as iodine. Maybe that's why I've been craving & eating it lately.