The previous article about this drink can be found here.
After a bit of hunting I finally found a couple of local places where I could find Izze Sparkling Juice on a regular basis.
I can see why they are useful in this context. They are unsweetened and are about 70% juice and 30% soda water.
If you wanted to do something similar a bottle of ruby red grapefruit and a splash of club soda would be pretty equivalent.
Using it here was a good, I used the same amounts as the original simply replacing the squirt with Izze. It still needed a dash of agave syrup to round the whole thing out. Without it they were bitter and sour without being tart.