As someone who carefully watches the grocery budget and tries to cook economically at home, I often look for ways to cut down on expensive pre-made drink ingredients and excess sugars. Regarding your question about finding a tart fruit for a smoothie, I believe that tart green apples or perhaps certain types of lemons might work well for this purpose, since they are usually reasonably priced at the supermarket. However, I am not entirely sure if they create a good texture on their own without adding something else to thicken the drink, because whenever I try making simple blended drinks, they sometimes turn out too thin or separated. You might want to experiment with a small batch first, but I cannot say for certain which specific fruit is the absolute best choice for a completely natural, tart flavor profile.
