Bountiful Gardens is now out of business, but you can get tree collard cuttings from Quail Seeds in California: tree collards This variety is exceptionally tender, especially in winter. It doesn't normally set seed, and has been passed from gardener to gardener for centuries. It was preserved by African-American gardeners and has been mostly unknown elsewhere until recently.
Quail Seeds also has some perennial vegetables that grow from seed, and seeds for non-hybrid, true-breeding comfrey. It is unusual to find the true comfrey species (Symphytum officinalis) that grows from seed---most of what people grow is the Bocking hybid which is sterile.
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop