Is there a store that sells herbal products?

If you mean local traditional medicinal herbs, the area around the Mercado Modelo in Santo Domingo is one option, or ask for your neighbourhood "bot?nica".

If you're after herbal or homeopathic type remedies, there are several places in the capital. Your best bet is probably Madre Tierra, Calle Padre Pina 2, just one block north of Ave. Independencia in Zona Universitaria - it's attached to a natural health clinic so they sell lots of natural medicines and remedies.
Tel. 809 686 7000.

There are other places, but this is the one I'm most familiar with.

Last Saturday I accompanied my wife, Altagracia, to a store fronting the back sidewalk of the Mercado Modelo, on Avenida Mella, Santo Domingo. While I was distracted playing dominoes on the sidewalk, she stocked up on several herbs from her favorite vendor, among the herbs I remember the following: pega palo, palo de cruz, paja de ajo, hoja tres palabras, rompesaraguey, anam?, artemisa s?bila, hoja de naranja agria, yerbabuena limoncillo, hinojo, albahaca, hierba luisa, salvia. In a neighboring store she also got the following products, camphor, bay rum, agua quita tormentos, agua siete viernes, among others. Please don't ask me what she uses them for... ;-)

In Cabarete, the Farmacia next to Scotia Bank has some american herbal/holistic type stuff, not much selection, but worth a shot if you're in the area.