To download a source image, go to the app store and choose dowload Marverick or Lion (not install) http://support.apple.com/kb/HT5855
./build_maveriks_iso.sh /Applications/Install\ OS\ X\ Mavericks.app/Contents/SharedSupport/InstallESD.dmg /tmp/
You need at least 16 GB Mount install.cdr as cdrom in your VirtualBox and boot The boot is slow and prints some errors but it works!
NB: when you first boot there are no disks available for install, you need to create a partion from the menu -> Disk Utility