I've got the Shen 6x17 back for 5x7 cameras. It's actually cheaper then the model for 4x5 cameras. It doesn't have the limits on focal lengths the 4x5 version has. If you have a 5x7 camera with the right sliders to take a rollfilm back then it's a lot cheaper then the Canham back.
The focal length discussions are always interesting. Seems half the people want wider then wide and the other half go longer. Check the various threads on pano cameras.