There's a few places you can go:
Edible Acres,
Food Forest Farm,
Forest Farm offer them bareroot whilst
Twisted Tree Farm sells open polinated seeds of Hungarian cultivars bred for timber production. The seeds from Twisted Tree Farm appears to be the only way to buy them if you live outside of the US since most nurseries that carry them don't ship internationally.
Note the owners of Edible Acres, Twisted Tree Farm and Food Forest Farm all work together and share material so it is likely that the germoplasm of all 3 comes from the same Hungarian cultivar(s) mentioned by Twisted Tree on their website. Forest Farm says that their material is sourced from the original wild "shipmasts" in Long Island.
