Every grocery store near us stocks it in a different place, so you may have to look around for it. I've found it on the....
--juice aisle with the kid's juice boxes
--baking aisle with cans of evaporated milk and sweetened condensed milk
--coffee aisle with all the creamers
I'd suggest buying some before you go and trying it out. My son never acted like it was any different, but some folks say their kids won't drink it because it tastes different. Better to find out before you leave....it's pricey (although very convenient!) and I'd hate for you to waste $$ on it! FYI...around me it's about $1.89 for a package of 3 8-oz. boxes of milk. LOTS more expensive than buying the half gallons at your resort...but you don't have to worry about refrigeration, so go with what works. We keep them on hand if we're going to be away from home all day or on the road traveling...just throw a couple in the diaper bag (with some clean, empty bottles) and we don't have to stop constantly to look for milk!
HTH,
Katie