You can buy the plant at the Jianguo Flower Market. I bought a small one last weekend actually, at the 3 for NT$100 herb shop. I think I’ve seen bigger ones at the two-ish regular herb shops as well.
(My previous one committed suicide before I could really use it.)
Some time ago you could find artificial sweeteners at Costco, the PX stores, and wellcome. Have not seen any available for a long time. Was having them shipped from over seas but because of my weight loss started using real sugar. I mostly go without anything these days. I can’t drink coffee without some sweetener but I’ve learned to drink tea without.