You can also try the freighters, like Solenta and FedAir. They generally use converted C208B's (passenger aircraft converted for freight by means of screening the windows) as opposed to the real Cargomaster, which has no windows in the airframe and thus no passenger seats. I suspect they might have some seats lying around, if not for sale then at least for rent.
Also, try Frost Air at Wonderboom, their C208 had seats, but most of it are removed at the moment.
Hope it helps!