It sounds like you're describing the same "seaweed salad" which is ubiquitous in japanese restaraunt in America. I love the stuff and would love to know how to either make it or buy it in bulk. I found this topic searching for information about it myself, ans just came back from searching for it.
It seems to cost the same whether you order it as an appetizer in a restaraunt or in the market. I found some in a Japanese market where I thought it might be cheaper or available in larger quanitym but it was in even smaller a size for the same price. So I bought some at the large american chain supermarket instead. The package lists the ingredients:
seaweed (amazing they can just say "seaweed" and don't have to be more specific, no?), agar agar, sesame oil, chili, sesame seeds, jelly fish, red pepper, FD&D yellow #5 (tartrazine), Blue #1 (brillian blue, FCF), citric acid, guar gum
I never would have guessed there was jelly fish in this, and really cannot tell what part of it this is (the clear, ragged-edged strands?). Besides the clear strands, there are at least 2 types of green (different kinds of seaweed?) and a small amount a black. Because there is dye in this, it makes it hard to tell what kind of seaweed in the store it might be.