I think many supermarkets now stock the Himalayan salt, but you can also try eBay or Amazon.
Re minerals,you may well need to watch potassium (which you've mentioned) and magnesium as well as your salt intake, as kidneys will probably be flushing out more of these minerals with extra fluid you wee. .Ckeck Google for vegetables (mainly green) that are high in these two minerals - there are actually better low carb sources than our bananas as I discovered! You'll find that many foods recommended for low carb eating also include their own built-in solutions to possible issues.( e.g.
@Rachox's seeds!)
Robbity
PS I've always used a fair bit of salt,but low carbing has actually reduced my blood pressure too, regardless.