I have never heard of any seeds being for sale directly from the U. When new seeds are developed by plant breeders at the U, I believe they are licensed to a seed company (I remember a squash or pumpkin that Burpee's sold) that would then sell it to the public -- either gardeners or commercial growers. The closest to direct sales I can think of is when the Research Center/Experiment Station with the apple orchard (now considered part of the Arboretum) sells bundles of scion wood to gardeners who want to try their luck at grafting specific varieties.
You may also want to contact the MN Landscape Arboretum to see if they sell seeds.
