I would recommend to visit a "Botanica", theses are small stores frequently located at the farmers market of the DR towns.
In santiago for ex. Hospedaje Yaque, Mercado Central and Mercado Modelo. Not sure if in the last one they still have as this has become to be more focused in selling souvenirs to the tourists who who are visiting the downtown of the city...
Los brujos , and los curanderos they know a lot about this!!!
JJ