Nearly all culinary salts contain yellow prussiate of soda to prevent caking. Most table salt also contains iodine as a dietary supplement. Natural or unrefined salt is recommended for lacto-fermentation. The higher concentration of other minerals in these salts provides nutrients to your product, rather than inhibiting fermentation as those additives do.
I use Redmond Real Salt purchased on Amazon. Due to the fine packaging it holds up well. It is well sealed and I have not had issues with it caking up.