Answer: Vigna caracalla is sometimes sold as Phaseolus gigantea. It's tropical in nature so you may be able to find it on the web at a nursery specializing in tropical plants.
I was able to find one nursery in California with Vigna caracalla in stock. Contact Exotica Rare Fruit Nursery, PO Box 160, Vista, CA 92093, phone 760-724-9093 or visit their website: http://www.bonusweb.com/exotica
Good luck with your new snail vine!
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