How can I make low sodium (140mg or less per serving) pancakes / waffles?
Reduce or leave out the salt.  It is not essential to the chemistry of the recipe.  I will not speculate on salt substitutes, as that is a health and medical issue, off topic for this site.

The pancakes will then not taste as good, but that is unavoidable.  Perhaps you can serve them with a highly flavorful accompaniment, like a reduced peach chutney or similar, to make the entire dish more interesting.

Not sure what kind of advice you expected.

Edit:  Based on the discussion of baking soda (sodium bicarbonate) usually used in pancakes:   as Kate Gregory points out, there may be alternate chemical leaveners.

Another possibility is to use mechanical leavening, by beating the eggs, fat, and any sugar well until they reach the ribbon stage (probably more than necessary, per Harold McGee, but still a clear indication); at this point they will have the maximum amount of air incorporated.   This batter cannot be held, but it should rise reasonably well, even without baking powder.

There are also recipes that use whipped egg whites to leaven pancakes, but they produce a different, fluffier product that may be called Fluffy Pancakes or Swedish Pancakes depending on the recipe.