Try Essene, they have lots of Japanese ingredients there, including kombu. I don't know whether the seaweed itself is from Japan or not though. I'm not sure where you're looking online but Mitoku brand is from Japan and is available online. They carry Mitoku products at Essene but I'm not 100% sure they have their kombu.

ETA: https://ediblephilly.ediblecommunities.com/shop/great-japanese-pantry-staples-essene-market, looks like they do carry Mitoku kombu at Essene.