I've found that the ... shape [of the camera] ... makes it somewhat difficult to use a prism finder.
I have only used the original series Exa with a waist level finder. Though I have never seen it mounted on an original design Exa, the standard prism should fit.
I have used, and still use when a nostalgia fit takes me, an Exakta/Exa 500, a later design of the concept. I find the chunky compact shape to be very pleasing, as do many reviewers.
http://www.cameraquest.com/exa500.htm
An Exakta 500 was my first SLR, so I guess I am imprinted with the design. Finding one isn't hard. Finding an unjamed one is. I ordered the fix-it manual and have to learn how to 'unjam' them - the manual devotes several pages to the issue.
Interestingly, the 'left handed design' of the Exakta Varex/1000 is actually designed for optimum usage by right handers - the reasoning being that the right hand should be used for focusing, aperature control and steadying the camera. It is all us right handers that have been stuck using left handed cameras.
http://www.wrotniak.net/photo/exakta/exakta-gallery.html
The Exakta 500 is ambidextrous.