Answer: Alfalfa is sold as seed, but to be honest I'm not sure you are thinking of alfalfa. It is a legume and grows two to three feet high and is usually used as pasture or mowed for top-quality hay. Another use is as a soil builder when grown as "green manure". People sometimes grow and enjoy eating fresh alfalfa sprouts, too.
Since it sounds like you are looking for something to help avoid mowing a bank, you might consider planting a permanent groundcover. If it is a sunny slope then low spreading junipers or perhaps a spreading form of thyme or even a flowering plant like daylilies (hemerocallis) or creeping phlox might be a good choice. If it is shady, you might like English ivy or vinca minor (creeping myrtle). Another legume, crown vetch, is sometimes suggested for covering banks, but in my experience it is more appropriate for the side of a highway or other waste site than for a residential setting.
If you really did mean alfalfa, Johhny's Seeds (http://www.johnnyseeds.com) offers alfalfa seed in quantity for field sowing and in small amounts for sprouting.
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