This list may help you, I've never been to any of them though. We go to Casa Toledo in San Cristobal, they have everything [fabric, trim, threads, rods, foam, zippers, you name it], but I'm sure one of these is similar.
For cotton fabric I would recommend RR All Cotton in Santo Domingo. They have a large selection of fine cotton fabrics. 829-654-1811. https://www.facebook.com/RRAllCotton/