Answer: You are looking for a hard-to-find mulberry that is not commonly carried in the nurseries. There are several varieties of mulberry which are sterile or fruitless, including Morus alba "Mapleleaf" with large, lustrous dark green, lobed leaves, and Morus alba "Stribling" which has good yellow fall color and appears to b quite cold hardy. Knowing the varieties should help you locate a source for these trees. Often local independent garden centers will special order for customers.
Forest Farm (www.forestfarm.com) sells a number of different varieties of mulberry, but I didn't see any fruitless ones listed in their current catalog. You might contact them and see if they can point you to a source.
As a last resort, you might try contacting one of these wholesale nurseries. They won't sell directly to you, but they might be able to point you to a retail or catalog source.
Angelica Nurseries is a wholesale nursery that sells Morus alba 'Stribling'
Greenleaf Nursery wholesales a fruitless variety
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