Unlike the more common smaller (not ULF) formats, whole plate holders are not of standard size. You need to measure your back to figure out what you need. Key measurements will be holder width and the distance from the "tape end" to the far side of the lock strip. Another key will be the "T" dimension, i.e. the depth from the front of the holder to the surface of the film/groundglass. That's a much tougher measurement to make, though.
What brand is your camera?