Conviasa is the only airline that has direct, non-stop flight between Port of Spain (POS) to Porlamar (PMV). From what I have found, it is also the least expensive, since any other route would turn this flight from 50 minutes to over 6 hours.

You can try Veneaven. After a long search, I was able to find this place that was able to sell me the flight from POS to PMV ticket at a discounted price. In fact, they offer very good discounted airfares from anywhere to Venezuela. The service is very good as well.

You can find them at www.venaven.com. They are also on facebook. Just search for venaven.