There are 2 established pool supply stores in Sosua. They are both at the east end of Sosua. I would ask to speak to the owner in private and ask him about who to hire to build your pool. When you find a pool builder, ask to see some pools they have built. Talk to the owners of these pools.
This is a big investment, a poorly built pool will make your life miserable. Take your time and be sure you are making the right decision.