Stumbled on this site while searching stores in England and it's LOADED with Clematis to buy in the US: http://www.joycreek.com/cl.htm#150-209-1.

Check out Silver Star Vinery. Email Debbie because sometimes she doesn't list ones if they are in short supply.
Haven't ever seen Mazury listed here in the US.

It wouldn't matter if you wanted to pay the price of shipping plants from England as the US has made it almost impossible to legally import plants from abroad without getting a phytosanitary certificate. They typically are not cheap!!

Here is a link that might be useful: Silver Star Vinery

I realize this is an old post, but in the Springhill 2009 catalog, they sell 'Mazury' for $14.99(just in case you're still looking)
Peggy

I might shy away from Springhill Nursery. They don't have consistent reviews so that makes me think that what you would get from them might just be a big toss up.

Here is a link that might be useful: Spring Hill Nursery on Garden Watchdog

I believe Springhill closed.

They are local to me and until about 15 years ago was one of the best local places to shop. They really went downhill when they sold out. *sigh*

I think you've got a handle on the best sources. None of my tanguticas bloom well except in full sun.Have you thought about trying 'Silver Moon', 'Rooguci' or 'Marmori'?

Those are my best shade bloomers. Good luck!

Mickie