This topic comes up every now and then, shooting square on an rb or rz. A good way to do it is with polaroid (I use type 55) on a quadra back, that's what I do. You get ~7x7 images. There are some square backs that show up on ebay every great now and then, but they are exceedingly rare. If you badly want it then you might need to make yourself a mask to go on your film holder. Or just crop.