At long last, here are some seeds rated for zones 4 through 9, and they're very inexpensive, too! I've never shopped with this vendor (so, I can't say what it's like).
http://www.treeseeds.com/white-mulberry-tree-seeds
Location: SW Idaho, USA
Climate:
BSk
USDA hardiness zone: 6
Elevation: 2,260 feet
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