Cornfed!
Lately I’ve been Looking for cold COLD weather trees.
The Russian Federation sells seeds on Ebay.. i have experienced at LEAST 60 percent germination (on average) and have placed several orders.
Sheffields, as mentioned, is great! Probably all you need.
If you shop for seeds in the fall, and they are “this year’s harvest”.. It doesn’t much matter.. the seeds are likely good. Can buy THEN from “randos”