The original Mendel? Not too many people work with that design since the improved Prusa version was designed. The reprap wiki page for the
prusa mendel has a link right to the github sources.
The parts are designed in openscad, but STLs are available which can be imported into many programs. Be prepared though, there are many different versions of some parts, and it's not well documented which ones are for which purpose. The design part is great for this version, organization of the github, not so much. For example, the x idlers and motor mounts for the lm8uu version in plate1.stl are different from the standalone versions in the same directory. Which one is better? I guess that's an exercise for the reader. In my experience it's a lot of trial and error figuring out which parts you want.