Most of the largest seaweed producers are unlisted in terms of public stock exchanges, such as Dalian Kowa Foods which is China's largest producer, or Seaweed Harvest Holland in Europe.

You could always start your own backyard seaweed production business:

"Meet the new US entrepreneurs farming seaweed for food and fuel"

https://www.theguardian.com/sustainable-business/2017/jun/29...