Varivas is it
A japanese company makes 8x-12x in their Midge tippet.
Their tippet is, IMHO, the best no matter the size. I'm not sure it is a "true" 12x. I believe there is some formula to determine the X rating that came from the old silk gut days and around 10x, the diameter is around 0. So a TRUE 10x is not even possible.
The 12x is, however the thinnest tippet available, regardless of x rating.
The 12x is a remarkable 1.06lb test. I think they use it for yamame fishing.
Do a search for varivas tippet for info. I'm sure one of our sponsors can get it if they don't stock it.
I make long midge leaders down to 12x and they are great fun for small fish of the sort I am often fishing for in little Pennsylvania streams.
I've also been thinking about trying to carp fish with 12x. Would be great for those super wary carp. And the test is so low that if I hook into one too big, I can easily break him off before he's fought to death.
I bought 8-12x and have been happily tying leaders with them for a ffew years.