You didn't specify the location or cities you are interested. My recommendation is to use Trulia's website to identify rentals. Then call the real estate office for the listing. An agent in that office can help you in your search if they are local to the areas you are interested in.
If you can't find what you need, there are rental companies which you pay a nominal fee to look at the addresses of the properties they have listed. You've probably seen their lawn signs. These have the most listings because they focus in rentals only.
Driving through the areas you are looking at can help identify places.