Here is someone else you might try: Julien Wilk. He's in France, but he was in Japan for quite a while. He runs the Link Removed. I've bought a couple box sets from. Much cheaper than Nick. I have no idea what he carries right now though
As you said, Muse discs are awfully rare although they do turn up now and again on Ebay. You might try Nick Santini, who is located in Japan. I bought my LD-S9 from Nick and a couple laser discs and was completely satisfied with his service. His prices, I will warn you, are pretty stiff. And...