Add this section to your /etc/apt/sources.list, the commented lines are the commands to issue to get the gpg key for this repository (without the hash of course (#)).
Code:
##################################
##### Kernel Archive Builder #####
##################################
#gpg --keyserver hkp://wwwkeys.eu.pgp.net --recv-keys 29F335B3
#gpg --armor --export 29F335B3 | apt-key add -
deb http://kernel-archive.buildserver.net/debian-kernel/ trunk main
Then do an "apt-get update" before trying again to install the headers.