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The least expensive and most practical solution to this request is to find a Kiev 88 prism in good condition. (probably on Ebay). They are well built,fit perfectly and are a solid performer optically. Can get from $50 to 80. I bought mine for $65 dollars and find it more than adequate for my needs on both my Hassy bodies.
I would buy the Kiev Spot TTL prism as well (US$ 90) from http://www.kievcamera.com/ if you live in USA or through ebay from Ukraine directly if you are from Europe.
I have a spare one left, though, as I moved to Rollei. Please note, Prisms add some weight to the camera. Calibration is easy. I would NEVER buy one w/o TTL.
--Rainer from Zurich
re: prism for the 500 c/m. go to www.keh.com. they have a wide selection of used hasselblad equipment. 30 return - no questions. i have bought frequently from them and returned things as well. their site ought to give you a clue.
I have the chance to buy a used PM5 prism viewfinder for my 500 C/M. Does anyone know whether the viewfinder enlarges the image as does the loupe in the normal viewfinder? This would be a very handy feature...