I don't know what happen to Sarah or her thread. I got here after reading her article too. My guess is the article was written some years ago.
After searching on the internet I have not found anyone who sells seagrasses. There are two prospects that you could call and ask. Gulf Coast Ecosystems and KP Aquatics.
https://www.live-plants.com/index.htm
https://www.kpaquatics.com/