When I owned the colmado we bought from the local almacen (warehouse), direct from the manufacturers (Presidente, Frito Lay, Helado Bon, Brugal), from butchers (chicken, pork, beef) from the local market (all fresh produce, tomatoes, fruit, onions etc) andthen from anyone who wanted to sell us produce. If you want to save money then buy at the local market or the local warehouse ( almacen). Both significantly cheaper than any supermarket and you don't have to buy in bulk. We would mark up as little as possible for the stuff the locals used; rice, oil, plantains, beans and more for what were considered luxury items like wine. Usually the beer and rum people tell you what to charge and the mark up isn't that much. International supermarkets usually seem very expensive to me, and to support the local area and the local people much better, healthier and cheaper to buy in the markets. Every town has a chicken killer (not sure what the pc word is) - so get your chicken there - fresh as can be and much cheaper.
Matilda