Yeh Paula, It is hard to get info from the rental, agencies. I have been searching for a house to buy for more than a year so I know what you are taking about. There are some that have websites and they are:
mirageproperty.com
alasmakhrealestate.com
remax-qatar.com
and soon there will be century21 too.
But even on those sites the don't list all the properties they have.......
Good luck
Aisha-Taweela