While it's called a limonada, you can use lemon juice or lime juice interchangeably. In Mexico, there are no lemons, just limes--hence the name limonada. You could also use half lemon juice and half lime juice or use orange juice to make a naranjada. If you do go with the orange juice, use about half of the stevia drops since oranges are much sweeter than limes or lemons.
lemon or lime juice
freshly squeezed (orange juice also works, but you would use about half the amount of stevia drops)
such as Perrier or Mountain Valley Spring
drops of liquid SweetLeaf stevia
8 drops for a tart limonada and 12 drops for a more balanced sweet-to-tart limonada
of lemon or lime