Metalliferous in NY, (212) 944-0909 , carries the larger sizes, all
the way up to 18.75 mm. I ordered some about 2 years ago. The
largest sizes, however, seem to be in fractional inches reather than
mm; not sure why.
Page 10 of their vol 3 silver catalog: think you can see it
Hope they still have it. It was the only source I could ever find,
and I really looked.