Hi Gillis and thanks for your post.
To be frank, there are not, to my knowledge, any agents who "specialize" in finding rental units for renters. Most agents handle all types of real estate related transactions from rentals to sales and purchases, so there isn't really "one" person who handles just rentals. And, unfortunately, there just isn't a lot of commission to be paid to a Realtor who finds a unit for a client to rent, but there is--in this particular economy--a whole lot of effort that has to be expended to find a rental especially when the client is not looking to rent a super expensive home or large estate.
However, you might find a bit more success if you work with agents or real estate firms who have an active business in managing rental units for independent owners. Look up real estate managers in the phone book or online and you'll be given the names of all the real estate offices that manage rentals. You might try giving a few of these a call to see if anyone within the organization can help you find a rental unit.
Because a good rental unit can be very difficult to find, the renter does have to spend quite a lot of time looking at units and negotiating with a lot of different rental managers and owners.