What's Fruit Salt?
You are looking for Sodium Citrate (citric acid salts). One of the most common brands of fruit salt is ENO, which is comprised of 60% baking soda and 40% citric acid. If you already have those two ingredients, then I suspect that you could substitute baking soda and citric acid for "fruit salt" (although I've never tried it). Alternatively, you could try substituting baking soda and some other souring agent in the same ratio.
Here is an explanation of fruit salt:
Invented in the 1850s by James Crossley Eno of Newcastle, the Fruit Salt sold like hotcakes to sailors looking for something to keep them healthy on long journeys. The product is still available today – now manufactured by GlaxoSmithKline, it sells in vast quantities worldwide and is a popular ingredient in Indian cookery. It contains sodium bicarbonate, citric acid and sodium carbonate, but in 1906 the Pharmaceutische Centralhalle für Deutschland analysed it as 50% sodium bicarbonate, 15% sodium bitartrate and 35% free tartaric acid.
But do click through the link - it also includes an advertisement for fruit salt from 1890.
Welcome to Stack Exchange! You should cover the information that the link leads to, in case the link becomes broken. As it stands, this answer is very poor. However if you added the info, it could be a very good one.
Apparently, this is a very old term (see page 360).
To answer your question, some recipes apparently allow you to substitute baking powder for fruit salt.
EDIT.. There may be a way to add cream of tartar to baking soda for a more accurate substitution. I mention this because I don't have sodium citrate on hand, but do have cream of tartar (potassium bitartrate). Also, a comment on this post indicates such a substitution strategy. When baking, the chemical balance may need to be accutate - wish I had a more accurate answer.