linux-headers-2.6.29-1-multimedia-amd64 download?
Currently running the above listed kernel version from the 64studio repository.
I'd like to build the fglrx kernel module using module-assistant, as a simple install of xorg-driver-fglrx yields
no DRI, only the 2D desktop. Googled the fglrx issue literally for hours with no resolution.
sudo m-a prepare tells me that the linux-headers for the running kernel need to be installed, but are not
installable.
Is there a reason why they are not available in the repository?

Instructions
Which installation instructions are you following?
Personally I have an nVidia card, and run the binary drivers i installed using this very kernel.
This looks like the guide I would try first if i had to install an ATI card today.
http://wiki.cchtml.com/index.php/Ubuntu_Hardy_Installation_Guide
I tried that installation
Kernel headers
Ok, was able to download and