Answer: The commonly named Scotch thistle or Cotton Thistle (Onopordum acanthium) is pictured here at the web site for Franklin Hill Garden Seeds, http://www.nb.net/~franklin/index.html and is available from them.
I found one source for Onopordum nervosum seed: J.L. Hudson, Seedsman, Star Rte 2, Box 337, La Honda, CA 94020 (no phone # listed). There are sources outside the USA--one is Bernwode Nusery in the UK which unfortunately sells plants rather than seed. (http://www.bernwodeplants.co.uk/home.html)
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