I've generally had good luck finding this at Asian grocers... It can be used as the base for the kind of Thai tea you get from a restaurant, just add sweetened condensed milk. I'm not a huge sugar fan so I drink it straight. Even then, it has a nicely bold flavor, and an incredibly high caffeine content.
Oh I've had this before, and it's so good! Definitely the best "Thai Tea" I've had at home that tastes like what I could get from a tea shop.
Adding it to my shopping list cause it's been too long (with some sweetened condensed milk ๐).