The source code of LuaTeX (and LuajitTeX) is hosted at Fabrice Popineaus place at Supelec (France): here. The sources and binaries of MetaPost and MPlib can be found: there.

For your convenience we have collected the latest stable binaries here. In order to prevent opening in your browser you can best click the right mousebutton and choose "save to disk."

windows  win32
linux   i586   x86-64   ppc
solaris  sparc   intel
os-x   intel   ppc   universal
freebsd  amd64   intel

If you install the ConTeXt distribution for your platform the right binaries and some support files will end up on your computer.