The Pentax A and M series right angle finders (refconverter) work with the K20d and K200d. I found one a couple of months ago at KEH. The K and previous versions fit the K series cameras and the M42 bodies whereas the M and A series fits M series and newer bodies.
Edit: People have replaced the eyepiece frame on some of the earlier versions of the refconverter and used the one from a stock eye-cup to replace the old eye piece frame. All of the earlier angle finders were not magnified.
Originally posted by kerrowdown Shame Pentax don't do interchangeable viewfinders for the more modern cameras, like they did for LX, we never had these sort of problems then.
The LX had a removable prism etc which is a whole different ball game.