I need to replace the window on the left seat door.  I cannot seem to order parts from Cessna so that I can replace the left door window.  The parts are so old that my mechanic is afraid to remove the assembly without having replacement parts.  Has anyone had a problem getting parts for oldies and found a way to circumvent the problem?  Are there alternatives other than Cessna to get parts.  Some have suggested a salvage yard, but, I am afraid that anything I get from salvage won't be any better than what I have now.  The plane is a 1968 172 I  s/n 172-56708