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.
README <https://github.com/Arquivotheca/SunOS-4.1.3#readme>
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.