Many local DR companies are involved in making shoe/boot soles and full-plastic sandals out of flexible PVC.
I'd expect demand of this material to be focused almost exclusively to the local DR market, as opposed to the free-zone exports market, as flex PVC is an inexpensive material with OK apparent quality, but with very poor long term performance and grip (i.e. you won't find Nike or Timberland using PVC, but there's a chance some free-zone manufacturers of slippers might be using the foaming flex PVC compounds for their low-spec products).
Here are a few contacts (wish we had an online DR phone book...maybe Mr.Emam-Zad? or someone residing there could post the phone#'s):
(In Santo Domingo)
Productos de Piel
Celso P?rez CXA
Calitex
There's also more than a couple local producers in Santiago that produce sandals and boot soles and trade regularly with Haiti.
Dominicus