After having huge Thai tea cravings, I decided I would try to make my own but couldn't for the life of me find a place in the GTA that sold Thai tea leaves until the internet led me to this place. The moment I walked in the door, the lady knew I was there for some Thai tea (Number One, currently selling for $6.50 a bag) and quickly got me what I wanted.
Just keep in mind that Vientiane is actually a warehouse, so it doesn't look or feel like a grocery store by any means. Also, like Bear stated, it's cash only! But if you forget, there's some business nearby where you can get cashback and atms.