Yes. You are right. Stevia is a natural form of artificial sweetener and considered way safer then canderal, sucral and other aimilar products.
You can buy stevia from al Fatah in Lahore, GNC and essajees have it too.
As for the organic stuff, you can find them at a store near mini market round about.
This store is all organic foods and stuff and its associated with an organization Scafa, the culinary arts school and fine dining place. Hope it helps.
As for online buying, not sure how authentic it is