Runners these days are using nuun tablets, I'm not sure if you can even buy salt tablets? Apart from salt-licks for cattle! Has anyone ever seen any? I used to be an EMT and I've never seen/used them or been trained in their use.
Not sure if you can still buy them, I suppose you must be able to, but they are far too dangerous to use without medical advise, bad for your heart and renal system. There is a massive difference between a trained athlete taking extra salt (after consultation) and just packing some in a survival tin, thats just plain fool hardy.
Just stick to the rehydrate drink powders, they contain other things your body needs apart from just salt.
I've just emailed an American pal who is over here working for the USAF (some of you met him last winter) and asked him to see if they are still issued, I'll post what I find out when he gets back to me.