Erfoud is well served by long-distance buses. The private ones leave from the main sguare, where the ticket office can be found. CTM ticket office & departures are on Bvd Mohammed V
Grandes Taxis depart from the square. This is probably the best way to move on to Rissani.
Its a soft, yet bumpy ride from Erfoud to the Saharan dunes. Noting but sand. The road peters out by and by. The drivers pick their way through the sand dunes while keeping the wheels moving. Don't want to get bogged down in the sand and have to push out. We were in a convey of seven four by fours. The drivers seemed to race each other as they fanned out across the desert scapes. Felt a bit like we were taking part in the Paris to Dakar endurance race. Thankfully, for the sake of my tender back the excursion was only about forty-five minutes each way.
Taxi are here like flies. If you need one you'll not have a problem. Some people pay to go to Marrakesh (750km), with this big bush cabs. They pay more than going by bus but if divided by 4 its much easier, faster and safe.
The Bus station is on the main square. You can buy tickets on the litlle ticket shop on side with the tea house on the corner.