Riding camels is widely advertised in the south eastern part of morocco. That is, where the Sahara starts. We might not be able to reach those parts. I am wondering if there are camel rides offered closer to Marrakech.
Marrakech has a big palm tree garden that seems part of the desert itself (well, like an oasis really). I don't remember the name, but it's really near the city center and every taxi driver knows where it is (just ask for camel riding). There are a lot of camels and people that offers camel riding to tourists. Also, there are travel agencies that offers camel riding experiences (one such is called clickexcursions).
For the 'proper' desert experience, many travellers head to the town of Merzouga and then onwards into the Sahara desert from there. Sure, you can find camels to ride in Marrakech but I reckon the experience won't feel the same as riding a camel in the desert. For that, I would suggest any of the tour agencies or even your hotel / riad that can arrange day trips to Ourzazate which is right at the edge of the Sahara desert. The shortest trips are just a few hours long: some sightseeing and a camel ride and then back. Longer tours include either a stay in a Berber camp in the desert, or if you prefer more comfortable abodes, inns along the road trip you'll be taking.
My recommendation would be get to Marrakech, and then book one of these short trips through your hotel. The hassle of negotiating with tour agents trying to rip you off in Djemma Al Fana is not worth it, and most hotels do reasonable prices anyway.
There were camels and their attendants on the beach in Essaouira when I was there ten years ago touting for tourists to take rides.
It's a lot easier to get to than the Sahara and it's a pretty chill place too. But it will just be a ride along the beach, which might or might be suitable since you didn't mention if you wanted to go trekking or not.
A company called Zouina-cheval has a website offering both horse and camel rides as well as trekking in Essaouira, complete with prices and useful details.
A page called "Essaouira Tourist Traps" on the site "Virtual Tourist" has people saying both positive and negative things about touts with camels in Essaouira, you might get some insights there.
You can find quite a few more hits by Googling "Essaouira camel rides".
Me and my boyfriend went on the Camel Trekking Night Desert Tour and had an amazing time. We booked the trip via email by contacting Hassan from Morocco Excursions company who welcomed us on the day of the trip. It was a long drive to get to Merzouga but it was well worth it. We were taken to the desert on camels and stopped in a camp for the night. There was around 10 other people with us so the company was great. Our guides prepared some delicious food and provided lots of fun and entertainment by playing drums and singing traditional Berber songs. We also had a chance to do sand boarding which was really cool. We slept in the tents, but there was also an option to sleep outside if you were not too scared off spiders and other various desert creatures:) We were woken up before 6 am and watched the sun rise. On our return to the hotel there was some breakfast waiting for us. We could also pick up our backpacks and had plenty of time to shower and refresh. I highly recommend the tour; the guides made us feel very comfortable and safe.