Well firstly what is the main reason you want the 0.1 preview source code? Because if you want it just to compile your own binaries for THAT RELEASE ONLY, it can be found in Sources/ inside the package of the game. (I'm going off the top of my head here, I don't remember if this is correct precisely... but it should be in the .zip file somewhere -- separated into engine source and game code source)
If you want to check out the latest code though, you should see the git repository which you can gain access to by following
this page on the development wiki.