I've never rented for the dunes. Usually along the vendor row in Buttercup & Glamis there's at least one vendor - Wish I knew the names. All I do know is they don't rent dirtbikes, dagnabbit!

You could try googling and see if anything comes up. I know there's places that rent out of Yuma. We are about 20 minutes from S Dunes and an hour to N Dunes.
Not sure if you've ever been to Glamis but be very aware of the weather. It is still in the 100's here. Pack lots of water! Always have a cell & if you have one a GPS. In fact if you don't have one consider buying one. Definitely plan on riding with a group that is experienced with the dunes - when you have dates set you can start a thread in the Connections Forum. Plenty of people are more than happy to welcome new members into their camps and show them around.