The Canham 6x17 back will fit 5x7" cameras that have Graflok-type slides (Canham, Wista, and increasingly others). It has motorized film advance, gets very good reviews, and costs about $1000.
I've been using one of the Chinese-made 6x17 backs that fits on a 4x5" camera (Da Yi). These cost around $550, and are a nice addition if you already have a 4x5" camera with a Graflok-type back, but are more limited than the Canham back in that they only work with lenses from 75mm to 150mm. Shorter lenses won't focus, due to the built-in extension of the back, and longer lenses vignette, though that may be acceptible (i.e., you could shoot, say, 6x15 with a 360mm lens).