Understandably, these may not be freely available, but I'm curious otherwise too (e.g. the likes of Air Crash Investigation must do this).
One place to look is models of the planes for flight simulators. There is generally a "sounds" folder of some sort inside the airplane folder that includes some cockpit sounds.
I'm guessing the original files are proprietary and there is really no reason for the companies to freely share them. Shows like Air Crash Investigation might get access to a simulator that can make all the real sounds.
I did a quick google search and this was a link that I found. Although, the quality of these samples leaves a lot to be desired.