Harding Ginseng farm may have goldenseal seed for sale. Just contact him via his website info. I know he stratifies a lot of it and may use it for his own planting. So I am not sure if he would sell goldenseal seed or not.
I do know that goldenseal seed is expensive. Seen it listed for $500 per 1/4 lb which is approximately 7,500 seed per 1/4 lb. Very expensive for sure.
I think most are reluctant to sell goldenseal seed as it can often be difficult to start a patch from seed and they do not want to be selling seed to customers knowing that it may or may not germinate. That is why most start patches from root stock verses seed. Root stock has great germination rates but as everyone knows root stock often cost more per plant than starting a plant from seed.
On the other hand, if seed is hard to germinate, then root stock makes more sense for starting a patch of goldenseal.
This is one of the reasons why I am willing to give the little bit of seed that I have collected away for free. Knowing it may or may not germinate which depends on many factors such as planting techniques, weather conditions, soil, canopy etc. Most importantly I like the thought of new goldenseal patches growing in multiple states from seeds that I give away. Wish I had more to offer on this subject. Good luck fellas.