You should consider whether sourcing finely ground salt would be easier than sourcing a high-quality grinder. In the US, "popcorn salt" is available and is an extremely fine grind. A grinder capable of a smaller grain size than that would likely be fairly expensive and difficult to source as a consumer, to say nothing of the humidity-controlled environment you'd need to use it effectively.
Though, for small amounts, a decent ceramic mortar and pestle is capable of grinding salt arbitrarily finely (until it absorbs water and clumps).