Where to find M50x.9 retaining ring for Xenotar 150?

Ok, so I acquired a nice 150mm f2.8 xenotar in barrel. However it lacks the retaining ring, which I believe is a 50mm x 0.9mm (M50x.9) thread. Anyone have something like that lying around or know where to find one off the shelf?

thanks!
-Ed

Have you tried S K Grimes?
http://www.skgrimes.com/products/index.htm

Thanks, I did try them, and ordered one from them, but it's a 6-8 week lead time.... hoping to find one in the meantime.