The DR is not up to the level of the US, England, and other first-world countries. Few businesses dealing with local residents have websites because few people have computers in their homes. There are many dealers of new and used cars, but you won't find them on line. Better to ask where it is recommended that you buy a car.
In my experience they are always late in posting/retiring new cars to/from their list. So only use it for a general idea and then call the dealers if you are interested. And read the classified ads in Listin Diario.