Ah, sorry! I missed where you said you were shopping in grocery stores. In that case, pre-made milk tea may not be possible, but
here's a relatively easy recipe that you can make at home using common asam or darjeeling tea (ie. black tea) which you can find in pretty much any grocery store. And the drink is very frou-frou, so I think it'd be right up your alley.
Also, if you ever make a trip to a co-op, Walmart, Target, or Trader Joe's (in the US - my apologies if you're from somewhere else), I've seen spearmint tea at those stores at times, though it might vary by location. Whole Foods may have it too, but I'm not positive. Otherwise, you can make it yourself by soaking fresh spearmint leaves in water; it works well with regular mint, too!
EDIT: Oh, I second Thai iced tea - it's delicious!! It's similar to Japanese milk tea, but it has a slightly sharper flavor. They have it in my grocery store in the Asian section. Try bubble tea, too if you haven't. It has chewy tapioca pearls at the bottom that are tasty and fun to eat/drink . That can also be found in most grocery stores, in my experience.