are you looking for a deflector that mounts to the box or cab? for the box I think any trailer repair shop would have one, or contact the maker of your box. for the cab I have no idea.
i am looking for one for the cab, that makes 2 of us, cause i have no idea, i called just about everyone i can think of, chev dealer was no help, so i dont know where else to ask!
Check with the Supreme corp. located in Elkhart, In they make the boxes for the cube trucks and am sure they have the air deflectors, or at the very least tell you where to get one http://www.supremecorp.com/
sounds like you want the wing some tractors have as opposed to a nosecone for the box? if its the wing i am sure a chrome shop will have one for a price