Solaris 10 kernel source was never released as open source. OpenSolaris started after the Solaris 10 release, using the source from the development branch which became Solaris 11. The work to separate the parts of Solaris that could be open sourced or had to be kept closed (due to third party rights) was never done for Solaris 10.
Solaris 10 kernel source was available for purchase from Sun under proprietary license terms, but I believe Oracle is no longer selling that either.