max@ubuntu:/usr/src$ ls
linux-headers-2.6.32-122  linux-headers-2.6.32-122-rtai

max@ubuntu:/usr/src$ sudo apt-get install linux-source
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  linux-source
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0B/5,260B of archives.
After this operation, 32.8kB of additional disk space will be used.
Selecting previously deselected package linux-source.
(Reading database ... 143721 files and directories currently installed.)
Unpacking linux-source (from .../linux-source_2.6.32.55.62_all.deb) ...
Setting up linux-source (2.6.32.55.62) ...

max@ubuntu:/usr/src$ ls
linux-headers-2.6.32-122  linux-headers-2.6.32-122-rtai
max@ubuntu:/usr/src$