where is my kernel source?

[Marlon Urias]

After going through pppconfig (after a fresh install of Debian 2.1) I
tried pon and got a error message about my kernel not supporting ppp.
So I head on over to /usr/src/linux to run make xconfig but alas, /usr/src
only has a dir named linux-x.y.z.headers (or something close). I was under
the impression that /usr/src/linux always existed. How can I reconfigure
my kernel under these circumstances? Do I need to download the source? Am
I looking in the right place? Help. Thank you.  marlon