On RHEL / CentOS, at least it's in the SysVinit package. You can spot it on
which reboot
to find the executable - it is in / sbin / reboot for me - then
rpm -qf /sbin/reboot
to find out what package it is in. You can then download SRPM from any CentOS mirror, for example. http://www.mirrorservice.org/sites/mirror.centos.org/5.5/os/SRPMS/
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