Although there are real estate companies & agents who also handle some rentals, you would be very limited seeking a rental that way.
The vast majority of rentals are handled by the owners themselves, or someone hired to do so.
I know this is not the answer you wanted and it makes it complicated, but that's the way that it is.
Shopping a rental on the net is really difficult.
You never know if it's a dump with a great website, or a great place with a lousy website.
Perhaps if you post exactly what you are looking for, we could make some good suggestions.
Here's a website you can look at for some condo rentals in Sosua.
http://sosuaproperty.info/
There is also a directory on DR1 with property rentals.
http://www.dr1.com/directories/index.shtml