kernel-source-2.6.15-27-server

http://packages.ubuntulinux.org/ is the best place to find official packages. Do you really want the source package or the binary one? There's only one source package as far as I know, not a separate one for the server. It's the same kernel, it just gets compiled with different options.

You can then search for packages from any of the releases. The following links to the linux-source package which is what I assume you mean?

http://packages.ubuntulinux.org/cgi-bin/search_packages.pl?keywords=linux-source&searchon=names&subword=1&version=all&release=all

Here's the link to linux-source-2.6.15

http://packages.ubuntulinux.org/dapper/devel/linux-source-2.6.15

Here is a link to where you can actually get the package:-

http://packages.ubuntulinux.org/cgi-bin/download.pl?arch=all&file=pool%2Fmain%2Fl%2Flinux-source-2.6.15%2Flinux-source-2.6.15_2.6.15-27.48_all.deb&md5sum=909f67c294d8b0490cf438f3fcf8dac9&arch=all&type=security

Thank you,
I just "moved from Sarge" and was convinced that name of the package should begin with "kernel-source" not with "linux-source".