There are a few companies that make dedicated covers, check the truck magazines. California Covers comes to mind, they make all types.
One word of caution however is to make certain you buy the best material available. If you are going to store the truck indoors with a cover that seems fine but some of the ones used outdoors in spite of the material can do more damage than good. The truck has got to breath under the cover and if it is located in a wet area there could be more damp problems. I have yet to see one used outdoors (car or truck) without problems. Of course I have not seen them all.
