This vape shop is by far the best: great selection and FAIR prices. Really good prices actually. The other places often try to charge 30-4-% more! I usually buy Naked 100 and my filters there.
address: go to the corner of Avenida las Carreras and Avenida Juan Pablo Duarte, then walk up Juan Pablo Duarte about one block. It is on the right.
Here is the Google Maps Link