I don't think it's difficult to find things in strange supermarkets while on vacation; just walk around the wall. In most supermarkets, most of the usual suspects for vacation food are near the outer walls. (Soft drinks, liquor, snacks, milk, bread, deli meats and cheeses.)
There are some things you should buy at home, though. Sunscreen is the most important; you need to be sure you don't have a reaction to an unknown brand, and different stores sometimes carry different lines. I would also suggest OTC medications, because they are time-consuming to find in a store, and usually overpriced as well if you buy small amounts. Condiments and detergents are best brought from home, because you can only take what you need, and most supermarkets only have packages that are too large for a week's worth.
Also, packages themselves, in the form of an assortment of ziploc bags; bring them from home and put all the rest inside one of the larger ones.
If there is some local delicacy that you really adore and must have, do bring it. Local specialties are not often found in resort destinations. I have been known to drag my child's favorite type of applesauce across three continents! Do try some local things if you get a chance, though -- you may discover something you like.