Salt shaker clogging is a big problem in the Caribbean- A older friend said his mom had these little Japanese porous tiles she add to her shakers. They worked great and could be re-used over and over for life. Does anyone know if they are still being made and if I can purchase them?
You may opt for purchasing coarse salt which, just prior to use, is milled to finer grains in a salt grinder. Their working principle is very similar to pepper grinders (torsion of the upper part into the opposite direction in respect to the lower part of the grinder), and thus equally sold in pairs with pepper grinders, too. They are available in different sizes, too.
(image credit to review of kitchen appliances).
Sometimes when you buy multivitamins there is a small packet of silica gel. I would say it is food-safe. I have been using one in my large salt canister for years. My salt shaker manages without one. If the humidity is really troublesome, you can remove the packet as needed and dry it out in a low temp oven for 30 minutes.
As this is the Life Hacks site (not a shopping site) I won't answer the XY problem but address the clumping or clogging of salt, which is the actual problem that needs to be solved.
Table salt used in shakers is different from cooking salt. Various anti-caking agents are used, such as calcium or magnesium carbonate, and sodium aluminosilicate.
If you are already using table salt, but the humidity is still too high for it to flow freely, there are other ways to absorb the moisture that salt attracts. This is to add grains or beans to the salt, that won't come out of the shaker's holes.
Long grain rice – large enough raw grains not to pass through the shaker holes.
Dried beans – white beans or kidney beans with a mild flavour won't affect the taste of the salt, and are large enough not to block the shaker's holes.
Coffee beans – will apparently not impart any flavour to the salt.
Cloves – will flavour the salt a bit, maybe to your taste.
I don't know which of those can be dried out and re-used when the shaker is empty (as you do with the porous tiles), but the coffee beans probably can, and the rice or the pulse beans could perhaps be used as an ingredient for soup or stew.
In conclusion I recommend you to try coffee beans, which probably don't need to be high quality and are inexpensive.