Hello, I have been looking for this clematis for a while. It doesn't seem to be in the trade in the U.S. I rarely see it offered in the seedex, but when it has been I've tried it without success. I'm interested in knowing if anyone grows it in the northeast or the Midwest where it is native.
It's native here in Alberta, and very common. I also grow it. I could send you some seeds if you PM me with an address. (By the way, the accepted name is Clematis occidentalis.)