So do you mean a topper? Never seen one for rent. Tons for sale cheap. You do have to make sure it fits YOUR truck. Even same brand over yrs have changed.
I see cheap/junky ones on lawns around. Quality ones go to dealership. Often if they buy truck with topper they try to sell it with topper for couple weeks, (at most) then remove topper and sell truck. I have seen some nice ones at dealerships/ reasonable (if you make a low offer)