Sorry if this is a repost. No idea of the legality, and therefore no idea how long it will stay: https://twitter.com/nixcraft/status/1586276475614818305 is the tweet and https://github.com/Arquivotheca/SunOS-4.1.3 is the repository with this README, below. Many other OS's there too.

This is the SunOS 4.1.3 SUNSRC CD-ROM.  It contains the source in 3 forms.

1. plain text source, as a ufs tree, rooted at the top level of
   this filesystem.  Symlinks to the SCCS hierarchy are in place.

2. SCCS hierarchy, rooted at SCCS_DIRECTORIES.

3. a tar image of the SCCS hierarchy, in a file named 4.1.3_SUNSRC.tar.
   This is rooted at ./SCCS_DIRECTORIES.

Please see the SunOS 4.1.3 Source Installation Guide for further details.