If you need a replacement for the interior door handles (Cessna® p/n 0517039-2), until recently your options have been limited.  It has been pointed out that aftermarket door handles from 1962 Mercury Comets will work on the Cardinal.  While it is true that they will fit, I recommend skipping that particular shortcut because the versions commonly available on the internet (~$26) have a cheap pot metal spline which will strip in about 5 uses.  This is not a part that you want to fail in the event of the necessity of a rapid egress from a burning airplane.  A replacement certified handle is available from Air Power Inc.  It has a different material in the spline location which should be more robust.  The price from them as of 28 April 2013, including shipping, was $189.65.  I bought one of these and it’s working fine.  Macfarlane also has a new design.