You can use morton salt as long as it doesn;t have phosphates (I believe).
You can buy it in huge bags. I can't remember hte exact type it has to be though, it might be NaCl? No that doesn;t make sense, why would you want chlorine ions in your tank.
I can;t think of it, just get the aquarium salt for freshwater fish though. I once had a petco guy tell me to put saltwater in the freshwater tank as its better than aquarium salt. I did not take his advice because they can;t keep even the hardiest fish alive.