I am not aware of any free spinning Inconel nut standards. The parts are primarily used in aerospace and all those nuts are locking design. Depending on how many parts you need, you could just contract with a machine shop to turn some nuts to the ASTM standard hex pattern out of some Inco 718. If you need a lot of them, you should be able to contact one of the aerospace nut makers and have them run some free spinning nuts; but don't expect to see them for about a year (especially from SPS).