Head to the Clinica Oftalmologica de Santo Domingo located at Calle Desiderio Arias 13 in Santo Domingo. They will take your insurance. They have a small shop with a good enough selection, but you can bring your own frames and they will fit the lenses. I did so recently and am very pleased, prices were much better for the lens than at Optica Felix, which has one of the largest selection of eye glasses here. You should consider seeing a doctor to get a preventive eye examination. Appointments are required.
Dr. Herbert Stern, the owner is a wonderful and very knowledgeable person. He is English-speaking. I believe several other doctors would also speak English.