Seaweeds and algaes can have a laxative effect. I tend to crave/eat kelp, dulse and other seaweeds when I'm low in iron. It's added to alot of foods as a thickening agent/emulsifier or flavour enhancer. I have to be cautious when consuming sushi, for example.
I generally buy it at either a Health food store or my local Asian market. It's also available in seafood markets.