Salt baths can be used for a variety of problems. Also, with a problem like limb rot, nobody really even knows what the causitive agent is (bacteria, fungus, virus), it's probably a combination of them. Any treatment is a shot in the dark.
You can buy kosher salt in most US grocery stores. It has no iodine or anti-caking additives, and it's usually in larger grains compared to regular salt. I have no idea what canning salt is, does it list ingredients? Any non-iodized salt is OK, including the aquarium salt they sell in pet shops. I've even heard that regular iodized table salt is OK to use.
