Ferreteria Popular has a good store on Ave. San Martin in Santo Domingo, several other good hardware stores there, including Americana, and some specialty places (i.e. different automotive, pumps, air conditioning, etc) too.. This is in the stretch between its intersection going below John f. Kennedy and heading west to Maximo Gomez.
If you finish there, and continue to the west, you will get to the larger Ferreteria Americana store between Lope de Vega and Abraham Lincoln on the right side of John F. Kennedy.