Kind of surprised there’s no mention of Fletchers’ Mill [0] here, they’re good quality mills made in Maine. The pepper mills use stainless steel while the salt mills use nylon (corrosion resistance), so you’d have to look elsewhere for salt.
Personally I just dump kosher salt into a salt cellar and call it a day but I am sure there are plastic-free salt mills out there somewhere.
Salt mills are sort of useless anyway… salt is just a bunch of little rocks, unlike pepper it doesn’t need to have been recently ground to be “fresh.”
Get some large sea-salt flakes if you want to be fancy with salt.