If you can't find the exact seaweed packets, I guess you can always use the regular rectangular ones in the grocery. You can just layer the tuna and rice and fold the seaweed into a square or a rectangle.
I can't say enough how convenient it is as a lunch-on-the-go meal because it literally is! It's a filling meal without the hassle of lunchboxes and plastic utensils. How cool is that? :)