You can try checking Horizon Herbs. I believe they may sell it.

FataMorgana

Here is a link that might be useful: Horizon Herbs

I brought 3 small plants of Ashitaba from Japan just recently. I hope to propagate them here in San Jose, CA.
At first, I tried to grow them from seeds but failed to germinate. I will update you.

Gabrielle2, it has been 2 years now, but has the ashitaba flourished in San Jose. I am in Fresno, would like to know