Shells are specific to particular truck brands and models, although there is overlap, of course. One which doesn't fit precisely (or is not installed correctly with the proper sealing) will leak and be no more protection than a good tarp job.

Just another thing to keep in mind while looking. A camper shell might also do the job, but I guess they have the same fit requirements.

If all else fails, Target sells good quality big plastic tubs for about $3 each. (The Rubbermaid ones leak -- don't get those.)