Hard to think of a substitute for ogo (it's nice and crunchy, a little briny)...
I've only seen in fresh in most of our supermarkets here, and is often used in poke, a dish of raw fish, cut into cubes and seasoned with a bunch of stuff.
One of the local companies here makes a poke seasoning thing in little packets, which looks like it has some dehydrated ogo in it.
I've never seen the dehydrated ogo by itself, but will look around to see if they sell it.