There are specialists such as
http://www.lawrenceofmorocco.com/about-us although each time I've been I have booked flights with the airline and then the Riad directly.
There are lots of Riads reviews on Trip Advisor and I would suggest having a good look there before selecting one.
A group of us stayed here
http://www.riadkarmela.com/home/ for four nights. It was a great location, they met and collected us from the airport for a small fee and even cooked when we required it. They also arranged taxis for us, a trip to an amazing spa in the Atlas Mountains and also booked various restaurants. I can't praise their customer service skills enough.
There are some amazing hotels too but they don't feel cultural whereas a Riad does. Again they vary in standards and prices.
Good luck