Cosmin, if the package you want is listed at the LuaRocks repository
(http://www.luarocks.org/repositories/rocks) you can grab the
.src.rock file, it is a zip with the rockspec and the tarball with the
sources.
I know, thanks, but not all of them have src rocks -- eg. ldecnumber didn't have one -- I'll make one and post it here- Doug Currie (the author) found me the sources.