Ok, to begin with all soda water contains sodium bicarbonate (bicarbonate of soda) usually along with a very small quantity of salt. In addition, depending on the manufacturer, soda water can sometimes also contain various other sodium or potassium salts to enhance the taste.
You can try making your own soda water from small bottles of sparkling spring water. What you need to do is to add one-eighth of a teaspoon of bicarbonate of soda to every pint of sparkling spring water. So how do you measure one-eighth of a Teaspoon? Well, you can use a cooks measuring spoon or get a large pinch of the powder out if the pot by putting your index finger, middle finger and thumb into the container and pinching your fingers and thumb together.
Drop the powder into a pint of sparkling water in a bottle, shake it up and see what it tastes like. Add a tiny pinch of table salt if you feel something's missing. Decant this into smaller bottles, even empty 250ml childrens lemonade bottles if need be. Making this a pint a time should allow you to retain a good deal of the carbonation. Make it as you need it and dont keep it for longer than 3 days. It's cheap enough to make this way.