what is the diffenence in the bed sizes between an OBS truck and a Super duty?

the reason i ask is i have a 97 f250 short bed crew cab. i have been contemplating putting a shell or a tonneau cover on my truck...however after looking at the cost of putting on a new one, 700-1500 dollars, i figured i would see what i could find used that may fit. have seen several tonneaus in my area off early super duty trucks, but i dont know if they will fit or not. the local tonneau and shell dealers all tell me that it has to be specific to the truck, but i think they just want my money. is there any other brand truck that has similar dimensions to mine? enough so that a tonneau might fit? thanks.