Sam, I think there is a little confusion over the terms "Rangefinder" and "Viewfinder"
RF : A Rangefinder is a device to help you determine the distance of the subject, based on the principles of trigonometry, using mirrors and prisms.
VF : A viewfinder simply helps you compose the scene that will be captured by the lens.
There are accessory RFs and accessory VFs on the market. From your original post, I can figure you are looking for a RF to help you determine the subject distance more accurately, so that you do not need to guesstimate the distance.
Actually no problem. With a bit of training and using the hyperfocal method(ie, large DOF, using smaller apertures 5.6, 8, or 11, etc), you can get pretty good results most of the time.