I have some ~3 year old
trees, it will be a while before I have seeds...
The seeds I planted I believe came from Twining Vine; I think they ship, but their site seems to be down at the moment. Don't recall if they specified a cultivar, or 60 vs 90, but seedlings survived -10C in small pots with no protection beyond a bit of mulch...
Hope that helps, good luck!
'Theoretically this level of creeping Orwellian dynamics should ramp up our awareness, but what happens instead is that each alert becomes less and less effective because we're incredibly stupid.' - Jerry Holkins