In many areas of the Caribbean
you will be lucky to find any milk at all in stores, and it will mostly be long-life stuff. Having said that, I wouldn't be surprised if you could find it in large supermarkets near tourist places, though I haven't looked. Basically, the rule
cruising is if there is some food
stuff that you absolutely have to have you must bring your own supplies of it as you can't be certain of finding anything when you need it.