How to attach elisp function source code in Emacs?

I could use M-x find-function to find the source code of some elisp functions if the function is not written in C code. But since I am using a **Debian binary package** of Emacs , some of the source code is omitted and only the byte-compiled .elc files preserved. So is there any way to get the source code and attach them with those elc files? Can I use apt-get source emacs and associate with the src (__C code__ as well) with the byte-compiled one. Or do I have to build Emacs from source code?