I may be incorrect, but I believe that you can only buy FXM's at knife shows or......on very rare occasions......when RHK has one in a lottery.
As N2K mentioned, they are all hand ground and Rick does not grind many.
In the last 12 months only one that I am aware of has come up in a Hinderer lottery.
I am not sure if any have been ground for shows but I would guess a couple have.
Your best bet, if you cannot make it to a knife show, is to keep an eye on the Exchange......a few pop up every year.......but like any other custom Hinderers......they can be difficult to find.......