Finding source repository for any command

Question

If I wanted to get the source for a particular command, for example, /sbin/shutdown. How would I go about finding which repo to add /etc/apt/sources.list and then how would I use apt to retrieve the code?

I know to retrieve the code, i can use "sudo apt-get source ???" but I can't find any resources on getting from /sbin/shutdown to the ???.

Enviroment: I'm using Debian Squeeze and kernel 2.6.32.