depends on how much you want to buy: small fixings and few thousand pesos cost and any ferreteria in sosua will do. a bot more stuff? i would go to ferreteria freddy in POP. house construction? directly in ochoa or bellon in santiago.
here, i called freddy for you:
cement bag: 275 (oriental) y 300 (cibao) pesos
cement blocks: tamano 6 (or was it 16? unfortunately at this very moment i spilled hot tea on my computer and had to go) 20.50 pesos