Check with Koh's Camera, too. They could have a used one. They have only charged me actual shipping for the small items I have bought from them-for instance, a padded envelope New York to California cost two dollars a couple of years ago.
You're going to have to start out with a diopter of some strength or other, so you will have the frame. As you know, the standard prism has a rectangular frame which is held in by a screw, not a round screw-in type like some other ETR-series prisms.