Unoffical repo with lua sources for windows dll build
If you want the official sources visit http://www.lua.org
If you want to build the lua5.3.0.dll yourselves you can find instructions here: http://blog.spreendigital.de/2015/01/16/how-to-compile-lua-5-3-0-for-windows/
The lua5.3.0.dll file can be found under artifacts.