Over Chuseok, I went to Japan (Osaka/Kyoto/Kobe Itinerary) and bought a ton of foodstuffs that I can't easily find in Korea.
I know that they do sell some milk tea powder in Korea, it's easily found at Olive Young, among other stores. The type they sell most readily is Hong Kong-style. I like that as well, but I am trying to reduce my trash these days, so the individual packets are not that appealing to me. Additionally, I like the richer flavor of royal milk tea vs. milk tea.
I bought this big bag at Don Quixote in Dotonburi, but I am sure you can buy it almost anywhere.
It's perfect for winter. The milk tea powder already has powdered milk in it, so I usually make it with water because that's what the directions say. I am pretty sure if you want a much richer flavor, you can use milk. It is sweetened, but not overly sweet.